In this commit, we make sig job handling when singing a next commitment
non-blocking by allowing the shutdown of a channel link to prevent
further waiting on sig jobs by the channel state machine. This addresses
possible cases where the aux signer may be shut down via a separate quit
signal, so the state machine could block indefinitely on receiving an
update on a sig job.
This is a requirement for replacing the quit channel with a Context.
The Done() channel of a Context is always recv-only, so all users of
that channel must not expect a bidirectional channel.
To make sure we attempt to read the results of the sig batches in the
same order they're processed, we sort them _before_ submitting them to
the batch processor.
Otherwise it might happen that we try to read on a result channel that
was never sent on because we aborted due to an error.
We also use slices.SortFunc now which doesn't use reflection and might
be slightly faster.
