geo-rep: Consume Changelog History API

Every time when geo-rep restarts it first does FS crawl using
XCrawl and then switches to Changelog Mode. This is because changelog
only had live API, that is we can get changes only after registering.

Now this(http://review.gluster.org/#/c/6930/) patch introduces History
API for changelogs. If history is available then geo-rep will use it
instead of FS Crawl.

History API returns TS till what time history is available for
given start and end time. If TS < endtime then switch to FS Crawl.
(History => FS Crawl => Live Changelog)

If TS >= endtime, then switch directly to Changelog mode
(History => Live Changelog)

Change-Id: I4922f62b9f899c40643bd35720e0c81c36b2f255
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://review.gluster.org/6938
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>