proposal: extend shm_mq to support more use cases

Hello! Apparently the current version of shm_mq supports only one sender
and one receiver. I think it could be very useful to add possibility to
change senders and receivers. It could be achieved by adding methods
that remove sender or receiver for mq.

As one of actual use cases can be some extension that creates several
background workers so backends can communicate with them. At the
startup the extension will create two mq for each bgworker (one for
input data, one for output). When a backend wants to work with the
worker it connects to one mq as sender, and to other as receiver.
After some working with bgworker it disconnects from mqs and the
worker becomes free for another backend. So the workers can act like a
cache, or keep some long connections with other services and so on.

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Ildus Kurbangaliev
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company