Add WRITE_SYNC flag to BagOStuff::set()/merge()

Add WRITE_SYNC flag to BagOStuff::set()/merge()

* This blocks on writing to all replicas
  and returns false if any failed.
* This is useful if ChronologyProtector is to work across
  domains by having the writes go everywhere so that later
  reads will see them (and be local at the same time).
* Redundant doc comments were also removed.

Change-Id: I9ed098d563c64dba605e7809bc96731da3b3e79d