server: explicit signal handler for SIGINT and SIGTERM

[ploxiln]

When running alone as pid 1 in a container, most signals that would
cause the process to exit by default, are instead ignored by default.
The container runtime will send SIGTERM, wait 10 seconds, then send
SIGKILL, which will work, but moto.server can exit much faster if
it has an explicit SIGTERM handler.

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An alternative way to fix the slow-container-stop problem is to use a separate "init" process, which forwards signals to the actual container command running as a child process (which then has PID != 1). You can get very easily get this with "docker run --init ..." which uses the "tini" init process included in docker ... but kubernetes doesn't offer this option, and the separate init process isn't really necessary.

---

If I could switch from werkzeug.serving.run_simple() to werkzeug.serving.make_server(), it would be more elegant: I could probably use the server object's log method and shutdown method (though I might need to spawn a thread for that too, long story). But only run_simple() provides convenient access to the auto-reloader functionality, it would be a lot of copied code to get that back for the make_server() way.

Anyway, I've tested by running this locally, it works :)

[coveralls]

Coverage decreased (-0.004%) to 94.61% when pulling 908fa1c on ploxiln:server_signal_exit into 02ac5ca on spulec:master.

[bblommers]

LGTM - Thanks @ploxiln!

[ploxiln]

Thank you for maintaining this useful tool :)