Rename close() to quit() and use promises for quit() method

[clue]

This PR renames the existing close() method to quit() and uses promises for this method instead.

// old
$connection->close(function () {
    echo 'closed';
});

// new
$connection->quit()->then(function () {
    echo 'closed';
});

This PR also includes relevant documentation and tests. The old close() method definition was somewhat inconsistent and under-documented.

Builds on top of #64
Refs #4

[clue]

For the reference: The method was renamed from close() to quit() to emphasize this method actually sends a "quit" command over the wire once all queued commands are done (it waits for all queued commands, which is also why it now returns a promise). It does not force-close the connection like the stream close() method commonly used in @reactphp does. I'm planning to add a new close(): void method which immediately force-closes the underlying connection and rejects all outstanding commands in a follow-up PR 👍

[jsor]

I'd personally keep the "return" value as void or is there a reason this now resolves with true?

[clue]

Good point! Updated to resolve with a void value instead. I'll make sure to file a similar PR which makes sure ping() also resolves with a void value instead of boolean true (refs #63).

[clue]

I'll make sure to file a similar PR which makes sure ping() also resolves with a void value instead of boolean true (refs #63).

See #66