handle_on_message is taking 250ms on prod

[robnagler]

It's likely there is some issue with the communication between the api server and the ui. At a minimum, the writing of messages should lock the process for 250ms. Not clear that threading (radiasoft/pykern#607) will fix this.

[moellep]

example warning:

pykern/pykern/api/server.py:215:log reply <subscribe#2 ui_ctx_update>
Executing <Task pending name='Task-67' coro=<_ServerConnection.handle_on_message() running at pykern/pykern/api/server.py:197> wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x7f44f5964bb0>(0] created at asyncio/base_events.py:429> cb=[set.discard] created at asyncio/tasks.py: 361> took 0.157 seconds