Improve cln plugin robustness

[JssDWt]

Main features

• whenever lightningd is gone or unreachable, the plugin will exit
• whenever the grpc server stops, the plugin will exit
• if the plugin can't log to cln, it will log to stderr
• the plugin will periodically log the number of inflight htlcs it is managing
• the cln interceptor will short-circuit htlcs that aren't forwards (the local node is the recipient)
• the plugin grpc server will reuse the port, even if it's already in use, to ensure it's always able to grab the port