Fix issues with idle message requeuing (#23)

* Fix issue with consumer deletion

Deleting a consumer causes its pending messages to get lost.
The code now uses a consumer pool instead of a 1:1 between sinks, streams pairs and consumers.
Consumers are marked as unused when correspding sinks are closed or go stale and creating a new sink attemps the reuse any unused consumer first.

* Handle idle messages and stale consumers

* Fix tests

Author: raphael

.gitignore

@@ -3,3 +3,4 @@ coverage.out
 examples/weather/bin/forecaster
 examples/weather/bin/poller
 examples/weather/signoz
+.DS_Store