MB-6714 Wake up the flusher after creating a checkpoint by force
MB-6714 Wake up the flusher after creating a checkpoint by force

Whenever a new checkpoint creation is requested from the external
components (e.g., consistent view support, XDCR synchronization),
we should wake up the flusher if its disk write queue was empty,
so that the flusher can update the last persisted checkpoint id
immediately.

Change-Id: Ib224aa5f57e4a6d7e5bec686d9da07bfe2001f0f
Reviewed-on: http://review.couchbase.org/21067
Reviewed-by: Michael Wiederhold <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>

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