Skip TAP cursor registration if backfill runs for a given vbucket
Skip TAP cursor registration if backfill runs for a given vbucket

If backfill is currently running for a given vbucket, we can simply
skip the TAP cursor registration because the backfill task already
sets the cursor to the beginning of the current open checkpoint when
scheduled.

Change-Id: Ifb36ccc5f21ba38520e6431bbdef4f3b81055984
Reviewed-on: http://review.couchbase.org/8180
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Reviewed-by: Srinivas Vadlamani <4036f57732a648e71da3ac2c829c8239a16a4c5d@couchbase.com>

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