Signals are not properly handled from another terminal #433

Doing ctrl-c in the terminal stops the turingdb properly (prints 'Terminating server'), but doing kill -2 pid or kill -15 pid only prints 'Terminating server' but doesn't actually stop the process.
This breaks the way we stop turingdb in the benchmarks.

The reason for this is that we handle the signal TuringServer effectively stopping the HTTP server main loop, but that doesn't stop the CLI loop.

@rjb32 @maxvincent24