Add function for checking async consumer event queue size

[ssams]

This adds a function consumer_events_available() on the async client to get the number of events available for consumption. Similar to e558946 I've added a default impl to the interface to avoid breaking old code which inherits from it.

The idea behind this is to allow exposing some stats in metrics, i.e. to see when consumers are lagging behind in consuming/processing messages, and the "pending" event count goes up. It may be also useful in some scenarios when implementing polling consumers, although I expect that in most cases the existing try_consume... functions are still more appropriate there.

[fpagliughi]

Good idea. One minor nit.

[ssams]

One minor nit.

If you're referring to the failed check, that seems to be some (temporary?) error in the Mosquitto installation before the actual build - I don't think I can rerun the check myself though? At least I couldn't find one so far...