Set a cursor to the current open checkpoint if backfill is required
Set a cursor to the current open checkpoint if backfill is required

If backfill is required for a given TAP connection, its cursor
can be simply set to the current open checkpoint instead of the
first checkpoint in the checkpoint manager, because items from
backfill actually reflects all the mutations upto the current open
checkpoint.

Change-Id: I94f3607782fb3ce345f46127639ac9b8d3c814b0
Reviewed-on: http://review.couchbase.org/6634
Reviewed-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

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