MB-11177: do not call removeOldFilesByVBid in getMulti

other minor fixes including passing strings by reference

Change-Id: I11b64df13a39f840a7ace05bde562f37c668d795
Reviewed-on: http://review.couchbase.org/37416
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>