Identify backfill completion for the client with no ACK support.
Identify backfill completion for the client with no ACK support.

If the TAP client with no ACK support connects to the server, the
server still should identify backfill completion and then send
any live mutations from the checkpoint to the client. From 1.7,
we don't allow backfill to be interleaved with live mutations.

Change-Id: I78a6e7c297dc569e68e14c57e169dadb6125c4d9
Reviewed-on: http://review.couchbase.org/6963
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Reviewed-by: Michael Wiederhold <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>
Reviewed-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>

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