MB-8464: Fix crash in test_checkpoint_persistence
MB-8464: Fix crash in test_checkpoint_persistence

- test_checkpoint_persistence seen to be failing
sometimes only in Linux due to a race condition
- adjusting the checkpoint_flush_timeout before
notifying_IO_completion during notify_checkpoint_
persisted seems to have stopped this error from
occuring.

Change-Id: I6804cb9dc566e9f0d2ed831c694feda19491762a
Reviewed-on: http://review.couchbase.org/27155
Reviewed-by: Michael Wiederhold <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>
Tested-by: Michael Wiederhold <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>

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