Add flag to use O_SYNC mode during rpc

RPC writes use buffered IO.

(There are some restrictions with DirectIO (aligned size, aligned buffers, file truncate))

On slow or bandwidth limited drives, if dirty writeback cannot catchup,  as a throttling mechanism, kernel (balance_dirty_pages) puts the process to sleep. 
Usually the delay is in order of 100ms but there can be a possibility that the process can get stalled/paused

https://access.redhat.com/solutions/899013 

https://elixir.bootlin.com/linux/latest/source/mm/page-writeback.c#L1925

With sync flag,  we can limit the generation of dirty pages which avoids this possibility as the writes are synchronous. Currently in plasma LSS files are opened using the same flag.

We already have a rpc config for this ((indexer.plasma.shardCopy.rpc.server.periodicSyncSize:0). From tests with the config, I did not see noticeable difference in perf tests that i have run