ep_engine pollutes log files on bucket creation

With these messages

Mon Sep 16 16:17:05.462500 PDT 3: (test) Warning: couchstore_open_db failed, name=/Users/artem/Work/couchbase/ns_server/data/n_1/data/test/975.couch.1 option=2 rev=1 error=no such file [none]
Mon Sep 16 16:17:05.462510 PDT 3: (test) Warning: failed to open database file, name=/Users/artem/Work/couchbase/ns_server/data/n_1/data/test/975.couch.1

This happens because during the warmup stage the vbucket files are not created yet and the following method is invoked: CouchKVStore::listPersistedVbuckets

There are couple of strange things I see in this method:
1. dbFileRevMapPopulated is always false. It can never become true.
2. discoverDbFiles check which vbucket files are actually exist but the following code ignores this information and blindly runs through all possible id's and tries to open db even on nonexisting files which results in bogus error messages in the log