MB-8903: Fake UPR server should support multiple sockets

Only the latest socket was used to mutations. Instead all
sockets should receive their corresponding mutations.

Change-Id: I857369e8469c3e610b8670fda279b3a63113314a
Reviewed-on: http://review.couchbase.org/30544
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>