Refactor not-my-vbucket handling into own function

This patch is intended to be cosmetic only, and should not change any
behavior

Change-Id: I647fc90056ef8ebe648aeaa37ad73e94a2fb651b
Reviewed-on: http://review.couchbase.org/29921
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>