Ensure graceful shutdown of `lnd` before electron quits

[valentinewallace]

Only quit electron when we've successfully received the exit signal from the lnd child process.

[tanx]

Closing due to inactivity

[cfromknecht]

@tanx can we reopen this issue? the app currently doesn't complete a graceful shutdown of lnd when exiting.

this was partially addressed by #550, which properly sends the SIGINT. however, when closing the app, the logs only show:

14423 2019-03-15 15:22:00.206 [INF] LTND: Received SIGINT (Ctrl+C).
14424 2019-03-15 15:22:00.206 [INF] LTND: Shutting down...
14425 2019-03-15 15:22:00.206 [INF] LTND: Gracefully shutting down.

So lnd received the request to shutdown, but since no other logs are found to me this indicates that the app isn't waiting for the child process to terminate on its own.

A graceful shutdown should result in the last line of the logs showing: Shutdown complete, like in this execution when I Ctrl-C the daemon directly: https://paste.ubuntu.com/p/7k4KBm4B8k/

[tanx]

@cfromknecht good catch! Yeah I've noticed that the lnd process keeps running in certain cases even when the app is closed.