WIP: start rewriting trait Subsystem

The goal here is to simplify implementation: instead of each
implementor needing to do the same right thing in `fn start`, we
now provide that method by default, and let the implementer just
provide the subsystem's name and its `fn run`. This shouldn't be
too big an adaptation as nearly-all, or all, existing subsystems
are implemented in terms of `fn run` already.

As a side-effect, the change in signature of `fn start` means that
we can now properly capture whenever a subsystem dies with an error,
and propagate that to a log at least. The overseer will restart it,
but we'll at least capture the fatal error in a log message now
instead of discarding it.