MB-27756 CouchRocks: Set the default value for 'bytes_per_sync'
MB-27756 CouchRocks: Set the default value for 'bytes_per_sync'

The 'bytes_per_sync' option allows the RocksDB background Flushers and
Compactors to incrementally sync files to disk while they are written.
One call to fsync/fdatasync will be issued every 'bytes_per_sync'
written. This setting helps to smooth write IO operations over time.

Change-Id: Ib6cbc995ecae0d3f1f9ebae729c5f2c59fb502ef
Reviewed-on: http://review.couchbase.org/88497
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine