snapshot: ensure cache is flushed before assume snapshot is ready

[maxux]

We can use power of 0-db for unlimited and automatic snapshot at any point of time.
But rollback to a time where some data are still in cache and are not in the database can lead to wrong rollback consistancy.

File system should have a way (probably an ioctl) to know a snapshot is requested. This would flush caches to ensure database is fully populated.

[sasha-astiadi]

not needed for next releasae qsfs 0.9