upr client: support multiplexed streams in fake server

Change-Id: I558dfb9723d72207067e12783b243a31e6453161
Reviewed-on: http://review.couchbase.org/34183
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
upr client: remove locally buffered stream events on stream_close

The server might have sent all the mutations to the client by the time
UPR_STREAM_CLOSE message reaches the server. Hence even if the server
replies with vbucket_stream_not_found, the client has to remove
internally buffered stream event queue if present.

Change-Id: I912dd190ebeb748534b58a6e9518f527792906a6
Reviewed-on: http://review.couchbase.org/34232
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>