Makes shutdown process asynchronous and graceful

[VividVisions]

As proposed in #84:

• EleventyDevServer.close() is now async and awaits all underlying close() methods.
• It also prevents multiple invocations (due to multiple SIGINTs).
• It also closes any existing WebSocket connections, since this doesn't happen automatically (see WS docs).
• Cliand Cmd handle the now async close().
• All tests also handle async close().

[zachleat]

Shipping with Dev Server v2.0.2, thank you!

(and core 3.0.0-alpha.18)