[Filestore] writeback-cache for files which are open for writing by only one client

[qkrorlqr]

If some other client tries to open the same file for RDWR as well, the tablet needs to notify the first client so that it would disable writeback caching, and only after that allow RDWR opening by another client.

fsync should cause cache flush of course

this issue will help with rsync-like access patterns - we noticed that rsync sometimes uses lots of tiny (1KiB) requests for writing large files - those files are usually opened by only one client, we can easily buffer those writes in filestore-vhost and send them to the storage in large chunks and with proper parallelism

the cache can be stored in a file over tmpfs (so physically - in the memory of the host where the guest is running)