Graceful shutdown of source providers

[seglo]

When a RunningProjection is stopped, call an (internal only?) async shutdown method in the source provider.

[ignasi35]

There are multiple reasons for a projection to stop:

1. the application is gracefully terminating
2. the projection is programmatically stopped (sharded projection moved to another ShardRegion) and may be restarted
3. the envelope handler errored (or there's a projection failure) and the projection will be (or should be) restarted

@seglo , did you have any of these reasons in mind?

[seglo]

@ignasi35 I didn't have any specific cause in mind. I only noticed that the projection has no way of signalling the source provider to shutdown except by way of the SharedKillSwitch. I was wondering if there was a way to use Alpakka Kafka's DrainingControl in non-exceptional shutdown circumstances.

[octonato]

I think this is a good idea. That would make it independent of the kind of kill switch is in use.