MB-9131 Warmup should check for couchstore dbfile before open

When creating dbFileRevMap during warmup, we now check for the
existence of couchstore db files so that we don't open non-existing
files so to avoid generating warning logs.

Change-Id: I8dd7b423f01240e8958b88fce2d58b429f0c9590
Reviewed-on: http://review.couchbase.org/40078
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>