Ensure that the millRun goroutine terminates when Close called.

[SimonRichardson]

This is an attempt to upstream and complete the patch that @howbazaar started a couple of years ago.

The motivation for this patch is to move back to tracking upstream rather than an independent fork.

The original description and PR https://github.com/natefinch/lumberjack/pull/100:

Currently the millRun goroutines leaks. This is very noticeable if a Logger is constructed periodically, used and then closed. This change ensures that the millCh channel is closed if it exists. Existing log rotation tests cover the duplicate Close calls.

[john8329]

This is a pretty important resource leak, @natefinch can you please take a look? I'm experiencing the same issue, no matter if I close the logger or not.

Also this is related https://github.com/natefinch/lumberjack/issues/205

Thanks

[john8329]

Also this https://github.com/natefinch/lumberjack/pull/57 this https://github.com/natefinch/lumberjack/pull/80 and this https://github.com/natefinch/lumberjack/pull/90 among others, this is getting worrisome, is this library maintained?

[xbtulip]

@natefinch This issue is mentioned by many users. Pls take a look.