Never retry node-specific commands

Items placed in the retry queue are rerouted based on their vbucket.
Commands which have no vbucket and thus may be sent to any server; or
commands which are forwarded to a specific server should never be
retried.

Change-Id: I1d6417e541dbcb0591a8bcdb2faa40d818e4aaa4
Reviewed-on: http://review.couchbase.org/42285
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>