Implement forest-cli daemon stop command.

[elmattic]

Issue summary

Forest is capable of shut-downing itself because it installed a handler that catches SIGINT and SIGTERM signals.
So if forest is running in the foreground, hitting Ctrl+C will stop the node.

We would like an equivalent of the Shutdown endpoint in common api of Lotus, as an alternative of resorting only on ctrl+c or kill commands.

Task summary

• Expose a Shutdown RPC endpoint with admin rights
• Expose some internal signal to gracefully shutdown forest
• Bind ctrc+c and RPC inbound message to it

Acceptance Criteria

• Calling forest-cli shutdown shutdowns the node
• Using signals (ie ctrl+c) still works

Other information and links

https://lotus.filecoin.io/lotus/manage/lotus-cli/#lotus-daemon-stop
https://github.com/filecoin-project/lotus/blob/master/api/api_common.go#L51

Note that this command will require that the RPC server be running.