In this commit, we start persisting shutdown info when we send the
Shutdown message. When starting up a link, we also first check if we
have previously persisted Shutdown info and if we have, we start the
link in shutdown mode meaning that it will not accept any new outgoing
HTLC additions and it will queue the shutdown message after any pending
CommitSig has been sent.
This commit adds an itest to demonstrate that the following bug exists:
If channel Shutdown is initiated but then a re-connect is done before
the shutdown is complete, then the initiating node currently does not
properly resend the Shutdown message as required by the spec. This will
be fixed in an upcoming commit.
