proper shutdown of holochain

[Connoropolous]

https://github.com/holochain/holochain/blob/12dfef0b2a710304697ddcd9ee731b4fe9933b0f/crates/holochain/src/bin/holochain/main.rs#L92

Since I would often find while spawning holochain as a subprocess that sending it a signal to kill it would not work, and the process would be left dangling, I wrote holochain-runner which is an alternative binary. For me, the ctrlc crate has worked well, with the feature called 'termination turned on which makes it call whatever shutdown handler you want when receiving the unix SIGTERM signal, and it also works a different way, but works on windows.

Since there seems to be cleanup happening I thought I'd mention this.

@thedavidmeister @freesig @maackle