The problem - we used file write in non-direct mode when writing snapshots in epoll mode.
As a result - lots of data was cached into OS memory. But then during the rename operation,
when we rename "xxx.dfs.tmp" into "xxx.dfs", the OS flushes the file caches and the thread
is stuck in OS system call rename for a long time.
The fix - to use DIRECT mode and to avoid caching the data into OS caches at all.
Fixes#3895
Signed-off-by: Roman Gershman <roman@dragonflydb.io>
