move subscribe/unsubscribe to client, for now

All the Web code that needs to subscribe to tables currently gets a
Client, not a Node, so this is easier. I might want to move it back to
Node in the long run.
DB: deliver NamedTuples to subscribers, easier to work with
rename test_db to test_database
web.py: overhaul EventSource API, add basic-message logger to control.js

Doesn't quite work yet, something about data.payload_json is wrong.