rgw: add preliminary support for sync update policies on metadata sync

We want to be able to conditionally apply new updates:
1) if we already have a newer version than the sync is applying for some
reason (replay of logs?), we don't want to go back in time.
2) If both zones were active at the same time, then we'd like to be
able to do a merge based on timestamps.

In order to support this, we add a sync_type flag to the implementations of
RGWMetadataHandler::put, and then check the version or the mtime of the
incoming put to what we have on disk, and refuse the update if needed

This commit does not complete the feature: we don't provide an interface
for specifying a different sync protocol.

Signed-off-by: Greg Farnum <greg@inktank.com>