Optimise io requests for long message queues

Ensure that the monitor reference is present in all receive clauses, so
that the compiler optimisation kicks in and the run time won't depend on
the length of the message queue of the calling process.

Remove the 'EXIT' clause, as its presence breaks the optimisation, and
that case is handled by the 'DOWN' clause anyway.

The test io_SUITE:io_with_huge_message_queue is an adaptation of
gen_server_SUITE:call_with_huge_message_queue.
Merge branch 'legoscia/io-message-queue-optimisation' into maint

* legoscia/io-message-queue-optimisation:
  Optimise io requests for long message queues