Replace GenEvent with alternatives

[davejlong]

GenEvent has been deprecated as of Elixir 1.5. Suggested replacements are Supervisor with GenServers or GenStage. I'm interested in trying GenStage as it's geared to be more stable, handling backpressure and multiple ways to handle the log output.

[davejlong]

Thinking about it, a decent solution, which could be a handy alternative to Elixir's Logger library is to refactor probe with GenStage. There would be 3 stages:

1. Event Producer (Probe.EventHandler)
2. Event Formatter (Probe.JSONHandler)
3. Event Writer (Probe.LogWriter)

This would enable Probe to be more modular in cases where someone might want to use an alternative log formatter, or alternative log writer. For example, instead of writing the log to a file, someone could replace LogWriter with LogstashWriter and write out to a log stash service.