Allow forwarding packets to specific servers

As a prelude to DCP support (and other extended commands), let the
specific server be determined beforehand, so the library doesn't need to
do an explicit vbucket mapping.

Change-Id: I78dbba27a75bd4e4dcb3a0f9bd04c49f47287a45
Reviewed-on: http://review.couchbase.org/42543
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>