Use asynchronous ssclient

[nzlosh]

Both btubbs/sseclient and mpetazzoni/sseclient use synchronous libraries under the hood.

The sseclient runs in a separate thread and listens to StackStorm's stream api. Both sseclient implementations work well until the err-stackstorm plugin is instructed to deactivate.

Both sseclients block waiting for events. In the case of mpetazzoni/sseclient the is no obvious way to signal the underlying event loop to stop and the client remains blocked until the process is terminated. In the case of btubbs/sseclient, the event loop executes at least every 60s when an empty event is emitted by the stream API.

To improve behaviour when the plugin is shutdown, an asynchronous sseclient should be used to avoid blocking.