This change adds a remote HEAD object request before each call to
sync_row.
Currently, container-sync-row attempts to replicate the object
(using PUT) regardless of the existance of the object on the remote side,
thus causing each object to be transferred on the wire several times
(depending on the replication factor)
An alternative to HEAD is to do a conditional PUT (using, 100-continue).
However, this change is more involved and requires upgrade of both the
client and server side clusters to work.
In the Tokyo design summit it was decided to start with the HEAD approach.
Change-Id: I60d982dd2cc79a0f13b0924507cd03d7f9c9d70b
Closes-Bug: #1277223
125d18e0ff