For ENG-5614, based on Ning's feedback avoid syncing from the write thread if possible

Make the sync frequency tunable by a property for experimentation. Also don't spin and sync unless the amount of data to be synced is at least 4 megabytes.
For ENG-5614, retrieve property correctly
Merge pull request #1046 from VoltDB/ENG-5614

ENG-5614: Snapshot syncing improvements