[Java] More carefully ensure sockets are closed in `NioPeerHandler`

[Java] More carefully ensure sockets are closed in `NioPeerHandler`

There is no indication that the sockets are leaking in the handling
loop, but its good to be careful and ensure we always both close
the SocketChannel and cancel the key registration.

During connection, however, it appears we may leak a SocketChannel
if the connection times out, leaking a file descriptor at least
until the GC kicks in and cleans up after us.  Here we are more
careful.