MB-3629 Pause TAP connection if backfill is required.
MB-3629 Pause TAP connection if backfill is required.

If the backfill is currently scheduled, pause the given TAP
connection so that the backfill thread can notify when
a memory or disk backfill is completed.

Change-Id: I07e7a2ce5672e627017273d7d0f40799f9865f02
Reviewed-on: http://review.membase.org/5457
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

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