Allow registration of threads with Pids

[andrewjstone]

System threads, such as server endpoints, filesystems etc... need a way to receive messages from processes. For instance, an API server receives a message, unmarshals it, creates an internal message, and sends it to the pid responsible. When the pid needs to reply with the message it needs a way to send it back to the API server so it can marshal a reply and send it to the client.

There are two ways to do this:

1. Hardcode the receivers in the processes and have them do the send on the channel directly
2. Register a sender keyed by Pid with the executor. Then return the reply to be sent from the pid just as with other envelopes.

We choose #2 since it enables easier testing and continues to not allow side effects outside the process.

For system thread PIDs I think we should make them part of the 'system' group in the Pid group field. This way the senders can be easily found by looking them up in a HashMap and also prevent the lookup of non system pids for normal envelope sending.

[andrewjstone]

Fixed by #3