MB-4971 Set the cursor to the open checkpoint in backfill begin.
MB-4971 Set the cursor to the open checkpoint in backfill begin.

If a given vbucket is empty, then backfill is immediately
terminated without setting the cursor to the open checkpoint.
This change fixes this issue by setting the cursor to the open
checkpoint at the beginning of backfill.

Change-Id: I67cf5bbcac905efe884ecf38f1fbef542cb84be8
Reviewed-on: http://review.couchbase.org/14402
Reviewed-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

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