server_negotiate: remove boilerplate user assignments

This commit allows the server_negotiate code to assign *itself* to the
connection object, so that its association is transparent to the
relevant bits of the calling code.

Change-Id: I52b399237caca78f82c04fab940d1978cd3cec4c
Reviewed-on: http://review.couchbase.org/34594
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>