MB-11190: Prevent deletion of valid vb file on warmup
MB-11190: Prevent deletion of valid vb file on warmup

Due to parallel warmup we have threads updating dbFileRevMap simultaneously
Take a snapshot before updating to prevent race where a valid file gets deleted

Change-Id: I1df6aebc4c1f51938477781b4c4ead06dd9cc545
Reviewed-on: http://review.couchbase.org/37484
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Tested-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

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