Simplify the checkpoint synchronization after backfill.
Simplify the checkpoint synchronization after backfill.

As we set a given tap cursor to the current open checkpoint when
backfill is started, we can simply start from that checkpoint
after the backfill is completed.

Change-Id: I12ee006b0376655eb2dc68a634fe5153f7d0658a
Reviewed-on: http://review.couchbase.org/6657
Reviewed-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

teligent-ru/ep-engine,sriganes/ep-engine,owendCB/ep-engine,daverigby/ep-engine,couchbase/ep-engine,hisundar/ep-engine,owendCB/ep-engine,couchbaselabs/ep-engine,zbase/ep-engine,sriganes/ep-engine,jimwwalker/ep-engine,couchbaselabs/ep-engine,zbase/ep-engine,zbase/ep-engine,abhinavdangeti/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,couchbaselabs/ep-engine,hisundar/ep-engine,daverigby/kv_engine,daverigby/ep-engine,daverigby/ep-engine,daverigby/kv_engine,owendCB/ep-engine,zbase/ep-engine,jimwwalker/ep-engine,membase/ep-engine,abhinavdangeti/ep-engine,teligent-ru/ep-engine,jimwwalker/ep-engine,couchbaselabs/ep-engine,membase/ep-engine,daverigby/kv_engine,abhinavdangeti/ep-engine,couchbaselabs/ep-engine,sriganes/ep-engine,couchbase/ep-engine,membase/ep-engine,daverigby/kv_engine,sriganes/ep-engine,zbase/ep-engine,membase/ep-engine,abhinavdangeti/ep-engine,daverigby/ep-engine,abhinavdangeti/ep-engine,jimwwalker/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,owendCB/ep-engine,hisundar/ep-engine