add ready? method to new plugin api

[colinsurprenant]

as proposed/discussed in elastic/logstash-devutils#32 and #3812 (now #3895) I am moving here the discussion about potentially introducing a new ready? method which would mainly help for testing to know when the plugin is ready and we can call the stop method on it.

there are a few things we should consider:

• it seems this new ready? method would only be used for tests/specs, in which case, is it really needed in a public api?
• it is hard de define the exact semantic of ready, is it ready once register has been called? or when the run method has been called? or when run has completed some internal initialization?
• in any cases, the stop method should always be callable after register has been called
• would the equivalent of the stop / stop? be sufficient? instead of a ready? we could just have a threadsaferun? method that just returns true when the run method has been called?

thoughts?