Extract the utility classes related with SSL to the util package
Fix a bug where SslHandler does not respect the startTls flag

- Fixes #856
- Add a dedicated test case: SocketStartTlsTest
Do not set 'Content-Encoding' if the target encoding is 'identity'

- Fixes #769
Make ChannelConfig.setAutoRead() triggers Channel.read() if autoRead was previously false

- also rename JDK socket and channel variables so that they are less ambiguous