Protect filesystem subtrees from concurrent manipulation during critical sections
if engine actions are called asynchronously. This change provides coordination
between the `Snapshotter` and the `FileWriter`. For example, the `FileWriter`
should be blocked from perturbing the same directory tree if a
Gather-Snapshot is taking place along that tree simultaneously.
This will ensure the fingerprint data accumulated during the `Gather` phase
will correspond unambiguously to the data included in the snapshot.
Extend build flags to kopia snapshotter
This package now imports fswalker which can only be built for
darwin,amd64 or linux,amd64
Add metadata R/W locking for asynchronous accesses to robustness engine metadata.
Remove the index from the Store interface and maintain it only in Checker, where it's used.
