Fix a potential bug where AbstractUnsafe.flush0() fails to clear the inFlush0 flag

- Fixes #1609
Remove unnecessary code from SslHandler

- Remove CloseNotifyListener which was used only to reduce the noisy logging.
- Instead, simply do a string match.
- Fixes #1608
Better promise validation messages
Even better promise validation messages
Merge remote-tracking branch upstream/$:{upbranch}