Separate lock for TAP backfill queue is not necessary
Separate lock for TAP backfill queue is not necessary

As backfill is not interleaved with live replication, we don't
need a separate lock for backfill queue. In addition, backfill
queue related variables should be synchronized through a global
lock per tap connection.

Change-Id: I655d01e5d30f0473325374fbe88f05ffbc848e4f
Reviewed-on: http://review.couchbase.org/6724
Reviewed-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

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