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 <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>