this set has to be protected by a lock. In ΔFtail it can be naturally
trated as "set of blocks that were explicitly requested to be tracked",
and protected by general ΔFtail locking. If it would stay in wcfs - it
would have to be protected by a separate lock and explicitly cared to be
synchronized with ΔFtail.

Change ΔFtail.SliceByFileRev to accept option to indicate whether we are
interested in changed covering only explicitly tracked set. Maybe in the
future we might want to make this the default and remove this option.

See changes in δftail.go for overview.

* t2+ΔFtail-concurrency: (39 commits)
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  X zdata: Switch SliceByFileRev not to clone Zinblk
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Zinblk is potentially unbounded large.
Should also help further steps on concurrency.