- ui_relay.ml: deal with Message.Subscribe and .Unsubscribe as well as .Post in api_tap_sink
 - web: add cache control information to served responses
 - use lazy and Lazy.force where appropriate

 - Figure out how to avoid the overhead of Message.message_of_sexp
    - straight matching of sexps without translation gives a ~5% speedup
    - checking for "post" first in message_of_sexp gives a ~3% speedup