Add infrastructure to buffer subscription events during directory cache
refresh. When a directory is being refreshed from the filer (BeginRefresh),
events from the subscription handler, local deletes, and local creates
are buffered. When the refresh completes (CommitRefresh), the filer
snapshot atomically replaces the cached entries, then buffered events
are replayed to ensure no mutations are lost.
This fixes a race condition where concurrent deletes or creates could be
overwritten by a stale filer snapshot during directory refresh, causing
ghost entries or lost files.
Fixes#8442
Chris Lu
58cf7e9a89