[Filestore] Compare sync vs async directory cache update options

[neihar]

During readdir logic, when we need read additional data we can update file based cache before or after reply to the guest

Updating after reply can cause rare missed cache data after crash which will lead to error during readdir.
If performance impact of sync approach is significant we need to reconsider our approach to unspecified behavior of readdir.

Currently behavior as follow:
If we read with offset, that wasn't acquired during consistent telldir:
case 1: inconsistent offset, but we have Cookie - do listing
case 2: inconsistent offset, but we don't have Cooke - return null (without error).

References #2390