Use coroutines for AlertMover and MonitorRunner

[ghost]

This change converts the MonitorRunner and AlertMover to use kotlin coroutines.

• MonitorRunner's blocking implementation and large thread pool have been removed without having to switch to a full callback style of API. We no longer allocate & block a thread for the entire duration we run a monitor. We no longer block a thread when performing I/O on the ES cluster (performing the search or reading/writing alerts). The only blocking I/O that remains is when publishing notifications which is done on a much smaller background I/O threadpool.
• AlertMover's callback based implementation is simplified considerably by coroutines.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[dbbaughe]

@vinooamzn Hi Vinoo, can you resolve the conflicts in this.

@jinsoor-amzn I believe you ran performance tests before for alerting to see the impact on a cluster, do you think it would be possible to run similar performance tests comparing alerting w/ threads and alerting w/ coroutines to confirm there is no negative impact for switching.

[dbbaughe]

LGTM, thanks for the changes Vinoo

[lucaswin-amzn]

LGTM, thanks for making this change! :)