(2.12) Filestore recovers from partial purge compatibility

[MauriceVanVeen]

This is a compatibility commit for #7676, specifically to solve this issue during upgrades/downgrades after hard kills while purge is being performed:

..., a hard kill on this new version that creates a __new_msgs__ directory with a tombstone, followed by a downgrade to 2.12 and more purge operations, and then a restart back to 2.14 would see the stream revert back to the sequences of that initial partial purge (since 2.14 would recognize that as needing to remove the msgs directory and replace it with __new_msgs__). The compatibility commit would purely be to remove the __new_msgs__ directory when seen, just like the purge __msgs__ directory.

Signed-off-by: Maurice van Veen [email protected]