Include detailed information on close outputs in listpeerchannels

[cdecker]

During close the status of a channel is rather complex to track. Many
of the outputs, whether they are direct to_us, to_them or delayed
to_us, to_them or HTLC sweeps, have different delays and may be
fully settled, settling or still waiting for a timeout to expire.

We should really have a list of outputs of the close transaction,
along with their state, value and a timeout in blocks until we expect
it to be resolved. This allows API users to determine the exact state
of the closing process, differentiating their various states.