Add Unix socket broker

[AlanCoding]

This is maybe, perhaps, a solution to #100 but it is debatable.

However, it was part of the vision of dispatcherctl described in #61

This was more complicated than I expected, but I got it working.

Using the "old" demo with 2 terminal tabs.

# tab 1
PYTHONPATH=. dispatcherd
# tab 2
./run_demo.py

It now get a bit awkward because you can no longer get a response from multiple servers. But we get to the end of the demo, and it prints everything. In my first pass it broke half-way through because the message was longer than 1024. I think I'm still putting in "reply_to" which the server just ignores.

Protocols need to be updated and basically everything aside from making it run the demo once, which is all it does right now.

[AlanCoding]

Oh, here's an interesting find from the demo output:

  Task-234 is done=False
   trace:
Stack for <Task pending name='Task-234' coro=<Broker._add_client() running at /home/alancoding/repos/dispatcher/dispatcher/brokers/socket.py:45> wait_for=<Future pending cb=[Task.task_wakeup()]> cb=[StreamReaderProtocol.connection_made.<locals>.callback() at /usr/lib64/python3.12/asyncio/streams.py:248]> (most recent call last):
  File "/home/alancoding/repos/dispatcher/dispatcher/brokers/socket.py", line 45, in _add_client
    line = await client.reader.readline()

It's right about where we expected it to be.

[AlanCoding]

Again, I hit an issue where it will be difficult to move forward before #121 lands.

When running commands as dispatcherctl entrypoint (the ultimate point of this), it can hang waiting for a reply. (yes this means timeout isn't working)

The reply isn't coming because origin information is 0. This is, predictably, because the control.py module has not had its contract updated to the same as DispatcherMain. However, the PR 121 removes the need to copy this contract in the first place. So that's where this leaves us.

[AlanCoding]

I figured out what it was. In asyncio, there are problems having parallel tasks both listening and writing to the same connection. This is roughly the same issue I hit with pg_notify. The solution is same in form, except for the multiple tasks for different clients.