154s -> 130s

    $ go test -v -failfast -run 'TestΔBTail$/rebuild'
    ...
    PASS
    ok      lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree        176.661s

--- FAIL: TestΔBTail (0.74s)
    --- FAIL: TestΔBTail/rebuild/T/T1/T-T2/B0:e-B1:f-B2:g→T1/T-T/B0:g-T2/B1:h-B2:g (0.00s)
        --- FAIL: TestΔBTail/rebuild/T/T1/T-T2/B0:e-B1:f-B2:g→T1/T-T/B0:g-T2/B1:h-B2:g/_T{};R (0.00s)
            --- FAIL: TestΔBTail/rebuild/T/T1/T-T2/B0:e-B1:f-B2:g→T1/T-T/B0:g-T2/B1:h-B2:g/_T{};R/_→T1/T-T/B0:g-T/B1:e (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x632cc1]

goroutine 238 [running]:
testing.tRunner.func1.2(0x696aa0, 0x893f70)
        /home/kirr/src/tools/go/go/src/testing/testing.go:1143 +0x332
testing.tRunner.func1(0xc000001b00)
        /home/kirr/src/tools/go/go/src/testing/testing.go:1146 +0x4b6
panic(0x696aa0, 0x893f70)
        /home/kirr/src/tools/go/go/src/runtime/panic.go:965 +0x1b9
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.PPTreeSubSet.ApplyΔ(0xc00040f9b0, 0x0)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/pptreesubset.go:515 +0x41
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.(*ΔTtail).rebuild(0xc00040f830, 0xc, 0xc00040f500, 0xc000142b40, 0x0, 0x0, 0x0)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail.go:371 +0x145
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.(*ΔBtail).rebuildAll(0xc00015dc00, 0x0, 0x0)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail.go:319 +0x18c
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.xverifyΔBTail_rebuild_TR(0xc000001b00, 0xc000142b40, 0xc00015dc00, 0xc00006a960, 0xc, 0xc0004863c0, 0xc00040f3b0, 0xc00040f5c0, 0xc00040f650, 0xc00040f740, ...)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail_test.go:1127 +0x1b6
lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree.xverifyΔBTail_rebuild.func1.1.1(0xc000001b00)
        /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/δbtail_test.go:1074 +0xafa
testing.tRunner(0xc000001b00, 0xc000555740)
        /home/kirr/src/tools/go/go/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
        /home/kirr/src/tools/go/go/src/testing/testing.go:1238 +0x2b3
exit status 2

Access to node entries is valid only after the node was activated.
And it fails otherwise.

--- FAIL: TestΔBTail (0.63s)
    --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d (0.00s)
        --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d/_T{};R (0.00s)
            --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d/_T{};R/_→T/T/B0:a (0.00s)
panic: BUG: T000000000000003a points to T000000000000000c as parent in trackSet, but not found in T000000000000000c children [recovered]
        panic: BUG: T000000000000003a points to T000000000000000c as parent in trackSet, but not found in T000000000000000c children

Rebuild tests were "surprisingly" failing when run as part of whole
testsuite, but succeeding when run separately.

So that even A \ A was not ø.

It was due to thinko in iterating through keys set - we were using key
ordinals instead of keys themselvs. Now it find such bugs:

=== RUN   TestΔBTailAllStructs/Update/T1,3/B0:d-B1:a-B3:d,4:g→T1,4/B0:e-B1:a-B4:c
    δbtail_test.go:839: 2: trackSet:
                have: map[000000000000000c:{pffffffffffffffff c2} 00000000000005c2:{p000000000000000c c0} 0000000000000618:{p000000000000000c c0}]
                want: map[000000000000000c:{pffffffffffffffff c3} 00000000000005c2:{p000000000000000c c0} 0000000000000618:{p000000000000000c c0} 0000000000000620:{p000000000000000c c0}]
    δbtail_test.go:708: T1,3/B0:d-B1:a-B3:d,4:g -> T1,4/B0:e-B1:a-B4:c  ; tracked={0 1} :

        d12:  map[0:{d e} 3:{d ø} 4:{g c}]
        δTok: map[0:{d e} 3:{d ø} 4:{g c}]
        δT:   map[0:{d e}]

        δZ:               {000000000000000c 0000000000000618 0000000000000620}
        Tracked^δZ:       {0 1}
        kadj[Tracked^δZ]: {0 1 3 4 9223372036854775807}
        kadj: map[0:{0} 1:{1 3 4 9223372036854775807} 3:{1 3 4 9223372036854775807} 4:{1 3 4 9223372036854775807} 9223372036854775807:{1 3 4 9223372036854775807}]

        δtkeycov wrong:
        have: {}
        want: {[3,∞)}
        δT    ∌  δTok[3]
        δT    ∌  δTok[4]

Currently fails with e.g.:

(neo) (z-dev) (g.env) kirr@deca:~/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree$ go test -short -v -failfast -run 'TestΔBTail$/Update/T3/B1:a-B8:c→T7/B1:a-B8:c'
=== RUN   TestΔBTail
2021/06/23 14:08:45 zodb: FIXME: open /tmp/δBTail926507346/1.fs: raw cache is not ready for invalidations -> NoCache forced
=== RUN   TestΔBTail/Update/T3/B1:a-B8:c→T7/B1:a-B8:c
    δbtail_test.go:812: BUG: tkeyCov1 ⊄ tkeyCov2:
                tkeyCov1: {[3,∞)}
                tkeyCov2: {[7,∞)}
                tkeyCov1 \ tkeyCov2: {[3,7)}
    δbtail_test.go:708: T3/B1:a-B8:c -> T7/B1:a-B8:c  ; tracked={8} :

        d12:  map[]
        δTok: map[]
        δT:   map[]

        δZ:               {000000000000000c}
        Tracked^δZ:       {}
        kadj[Tracked^δZ]: {}
        kadj: map[1:{1 8 9223372036854775807} 8:{1 8 9223372036854775807} 9223372036854775807:{1 8 9223372036854775807}]

--- FAIL: TestΔBTail (0.84s)
    --- FAIL: TestΔBTail/Update/T3/B1:a-B8:c→T7/B1:a-B8:c (0.00s)

=== RUN   TestΔBTail/Update/T7/B1:a,2:b-B8:c,9:d→T3/T-T/B1:a,2:b-B8:c,9:d
    δbtail_test.go:832: 2: trackSet:
                have: map[000000000000000c:{pffffffffffffffff c2} 0000000000000041:{p0000000000000043 c0} 0000000000000042:{p0000000000000044 c0} 0000000000000043:{p000000000000000c c1} 0000000000000044:{p000000000000000c c1}]
                want: map[000000000000000c:{pffffffffffffffff c1} 0000000000000041:{p0000000000000043 c0} 0000000000000043:{p000000000000000c c1}]
    δbtail_test.go:708: T7/B1:a,2:b-B8:c,9:d -> T3/T-T/B1:a,2:b-B8:c,9:d  ; tracked={1} :

        d12:  map[]
        δTok: map[]
        δT:   map[]

        δZ:               {000000000000000c 0000000000000043 0000000000000044}
        Tracked^δZ:       {}
        kadj[Tracked^δZ]: {}
        kadj: map[1:{1 2 8 9 9223372036854775807} 2:{1 2 8 9 9223372036854775807} 8:{1 2 8 9 9223372036854775807} 9:{1 2 8 9 9223372036854775807} 9223372036854775807:{1 2 8 9 9223372036854775807}]

        δtkeycov wrong:
        have: {[7,∞)}
        want: {}