data sync options for STORE

[tomcucinotta]

The STORE command has been sketched out with the idea of a per-request sync, with the ODIRECT option specifiable for the whole node as command-line option to dw_node. We'd need to add more options in this regard, to mimic common syncing behavior of a multitude of systems (ideally, still under control of the client):

• no syncing at all
• per-request data sync
• periodic syncing of written changes (e.g., common of DBs, MongoDB/WiredTiger, Cassandra, ...) with customizable period

[tomcucinotta]

still missing the "no sync" option, to close this; not sure if it makes sense to distinguish:

• writes that need an immediate sync before getting a response
• writes that do not sync immediately, but will wait for the next periodic sync to have been completed, before getting a response (i.e., when the periodic sync happens, we need to track what req_ids are being synced)
• writes that simply write() and respond immediately to the client.