More sophisticated electrumsv-sdk "stop" command

[AustEcon]

With the introduction of the StatusServer (and websocket) coming soon... I think the "stop" command needs to become more sophisticated to specify which ComponentType(s) it wants to stop.

By default (no args) it kills all processes that were started

I think instead it would leave the status server running (but kill the rest) until it is explicitly stopped via:

electrumsv-sdk stop status_server

or

electrumsv-sdk stop all

Currently the stop command just iterates through all of the ComponentTypes that it started and kills them all indiscriminately.

Instead each ComponentType (with pid attached) should be looked up and killed selectively depending on arguments to the "stop" command.

This allows the front-end to maintain the 'link' of being able to check the status (with the default usage pattern)

[AustEcon]

resolved in 2849e94

However, this needs to be made more granular to address requirements for:
#16