upr client: Use queue module instead of list based queuing

Replaced expensive list based FIFO implementation with
erlang queue module

Change-Id: I112fe5dafe990748fa75fd1a321ea39d94312c97
Reviewed-on: http://review.couchbase.org/34645
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>