Start the gen_server from handler_added instead the other way round

[peerst]

And store the pid in the handler, also shut down the gen_server if the handler is removed.

• [X] Handlers should be added by users and not by the application
• [x] Start and shutdown a separate process for each handler

[eproxus]

Should be started here: https://github.com/grisp/dgram_logger/blob/master/src/dgram_logger.erl#L20 Then socket can be requested and put into Config.

[eproxus]

Handlers are no longer added as from 9476009726ceb401049f12c1d3e5801cebbb303b.

[eproxus]

Separate process and socket started for each handler added in 1067301a97f7dccc7d22c2aabb04d517adbda146.