Made isOpen() public to avoid clashes with AbstractEndPoint.
403451 - Review synchronization in SslConnection.

The review consisted in finding all the places where SslConnection was calling application code
and made sure those call where performed outside synchronized blocks.

The few calls that remain within synchronized blocks are SslConnection.fillInterested() and
SelectChannelEndPoint.write(...), which are dealing with the encrypted connection and do not
call application code.