Step №2 in making htlcManager (aka channelLink) testable:
Implement the ChannelLink interface which is needed to use it in pair
with htlc switch. With this commit channel link impelements interface,
but isn't able to operate properly yet.
In this commit all initial code which will be transformed into channel
link have been added. Rather than changing the in the same commit is
better to create the standalone commit, in order to see the changes
which have been applied to relocated code.
