Create a health check for the Extempore TCP server

[oubiwann]

A positive is fairly easy to do: send a blocking message to the server and if a result comes back, you're good. Simply sending a bool like #"#t\r\n" would do it. However, if the server is not okay, you can be in a bad state. A timeout will help with that (see lfex/tcp-client#7). Until then, an async message that writes a warning to a log should be fine.

This could be done with sending a specific bit of data, e.g., #(health ok) which will then be resent by Extempore back to undertone. When that specific packet comes back, it can be intercepted and a lot message can be generated.

Tasks:

• Create a (check-xt) REPL function, which will async send #(health ok)
• Create a new pattern check in the TCP message handler for #(health ok), logging that at notice-level

[oubiwann]

Done.