MB-100: disable stats collection during load and index phases

Change-Id: Id5b53873ebfaac09b4c7bf3fbf97b64ec037000e
Reviewed-on: http://review.couchbase.org/25559
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>