Graceful shutdown

[cressie176]

Currently shutting down the rascal broker will

1. Unsubscribe all subscriptions
2. Disconnect from all vhosts

This means any in-flight unacknowledged messages will be rolled back (and redelivered), and the outcome of any inflight publications is unknown.

It will never be possible to shutdown in a completely graceful manner since Rascal is not responsible for the full unit of work, but it should be possible to delay disconnect until

1. All in-flight messages have been ack'd/nack'd (or a timeout expires)
2. All borrowed publication channels have been returned to the pool (or a timeout expires)

The former would require counting messages in and out. The latter requires setting the channel publication pool to drain after pausing channel allocation.

Thoughts @BorePlusPlus ?

[cressie176]

The next release of Rascal drains the publication channel pools and waits a configurable period of time between unsubscribing and disconnecting to allow inflight messages to be ack/nackd