build logs are stored in an events table

...rather than one gigantic column. there's a tricky migration here, but
it should be self-contained at least.

[#80512764]
zero-prefix migration files
switch to server-sent events for all build logs

* the scheduling ATC is now responsible for tracking each running build,
  by consuming events from its Turbine endpoint
* remove callbacks endpoints; they are no longer used
* in-flight builds proxy events to their turbine; completed builds read
  events from the database
* remove logfanout and a bunch of no-longer-relevant code
* if a running build's turbine reports 404, mark the build as errored

[#72547842] [#80512764] [#80649160]