which is showing itself as e.g.

---- 8< ----
panic: Buf: refcnt < 0

goroutine 7 [running]:
lab.nexedi.com/kirr/neo/go/zodb.(*Buf).Release(0xc42000db40)
        /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/buffer.go:107 +0x116
lab.nexedi.com/kirr/neo/go/zodb.(*Buf).XRelease(0xc42000db40)
        /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/buffer.go:136 +0x3a
lab.nexedi.com/kirr/neo/go/zodb.(*Cache).gc(0xc4200101c0)
        /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/cache.go:488 +0x22f
lab.nexedi.com/kirr/neo/go/zodb.(*Cache).gcmain(0xc4200101c0)
        /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/cache.go:451 +0x3f
created by lab.nexedi.com/kirr/neo/go/zodb.NewCache
        /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/cache.go:123 +0x126
---- 8< ----

during zhash.go -prefetch run.

This patch adds the test for this bug and explains the problem in detail
there. The test currently fails:

--- FAIL: TestCache (0.01s)
panic: Buf.Incref: refcnt was < 1 [recovered]
        panic: Buf.Incref: refcnt was < 1

goroutine 18 [running]:
testing.tRunner.func1(0xc4240ce000)
        /home/kirr/src/tools/go/go/src/testing/testing.go:711 +0x2d2
panic(0x588a20, 0x5ebe70)
        /home/kirr/src/tools/go/go/src/runtime/panic.go:491 +0x283
lab.nexedi.com/kirr/neo/go/zodb.(*Buf).Incref(0xc4200c2460)
        /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/buffer.go:134 +0x58
lab.nexedi.com/kirr/neo/go/zodb.(*Buf).XIncref(0xc4200c2460)
        /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/buffer.go:148 +0x3a
lab.nexedi.com/kirr/neo/go/zodb.(*Cache).Load(0xc4200ca000, 0x6ab1a0, 0xc42001a148, 0x4, 0x1, 0x10, 0xc4200bf2e0, 0x10, 0x10)
        /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/cache.go:168 +0x1c7
lab.nexedi.com/kirr/neo/go/zodb.TestCache.func2(0x4, 0x1, 0xc4200c2440, 0x4, 0x0, 0x0)
        /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/cache_test.go:155 +0xcc
lab.nexedi.com/kirr/neo/go/zodb.TestCache(0xc4240ce000)
        /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/cache_test.go:579 +0x4eb5
testing.tRunner(0xc4240ce000, 0x5d4728)
        /home/kirr/src/tools/go/go/src/testing/testing.go:746 +0xd0
created by testing.(*T).Run
        /home/kirr/src/tools/go/go/src/testing/testing.go:789 +0x2de
exit status 2