Only log once when the backfill token becomes invalid
Only log once when the backfill token becomes invalid

Change-Id: Ib346c7023700a3af38481511543841bd2e7ed9bc
Reviewed-on: http://review.membase.org/4229
Tested-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>
Reviewed-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>

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