bug 1696585: switch to honcho for eliot

[willkg]

This switches Eliot to use honcho for process management. We're going to
standardize on honcho for cases where we need to run multiple processes
in the Docker container.

[willkg]

After this lands and deploys to stage, we should check the logs to make sure logging is still working.

To test:

1. run docker-compose up eliot to run eliot

In the output, it'll print this:

eliot_1         | /usr/local/lib/python3.9/subprocess.py:941: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
eliot_1         |   self.stdout = io.open(c2pread, 'rb', bufsize)
eliot_1         | /usr/local/lib/python3.9/subprocess.py:941: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
eliot_1         |   self.stdout = io.open(c2pread, 'rb', bufsize)

That's covered in this Honcho issue: nickstenning/honcho#221

That's fixed, but not released, yet.