ConnectionAdapterImpl is always bound to a channel

ConnectionAdapterImpl needs to have a channel -- so require it in the
costructor and make it final.

Change-Id: Id170afd055460c24b851cdb3969286c62eee22d0
Signed-off-by: Robert Varga <rovarga@cisco.com>
Signed-off-by: Michal Polkorab <michal.polkorab@pantheon.sk>