Avoid getting config lock on every unlinkCouchFile

Change-Id: I7a5c3cf81a227ee2ddc02750839cc3b94359fe88
Reviewed-on: http://review.couchbase.org/41004
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>