Use ReadTimeoutHandler for IdleHandler

This converts to using ReadTimeoutHandler, as it has no impact on the
writeout path -- which IdleStateHandler does by attaching listeners to
promises flying by -- which is omething we don't need.

Change-Id: I4e6f4ac862747116aa9f5f4fcaa9fc9d562f7de9
Signed-off-by: Robert Varga <rovarga@cisco.com>