Major rework of command submission + task events

[rdaum]

Rework the $do_command -> parse command cycle

• Moves the logic for running $do_command out of the RpcServer and
  into the task itself.
• It's a big gronky, but I wrote tests. And the tests pass.
• Removed the blocking watch command piece from rpc_server.rs, so
  another means is going to be necessary for reporting failures back
  to the host.
• Major reworking of the RPC layer to handle async events:
  • Errors from task submissions are now handled asynchronously,
    published out to clients as they complete, rather than blocking
    the rpc server (& telnet connection)
  • "narrative" pubsub stream now becomes "events", more generic
  • Moved SchedulerError and friends up into model, & cleaned up the
    Rpc error enum to use that instead of duplicating a bunch of
    values.
  • All tests pass, JHCore runs.