* Test working through a backlog of change events
* Use a slightly more performant threaded http server in eventing pytests
This helped locally but not on CI it seems...
* Rework event processing for backpressure. Closes#3839
With loo low `HASURA_GRAPHQL_EVENTS_FETCH_INTERVAL` and/or slow webhooks
and/or too small `HASURA_GRAPHQL_EVENTS_HTTP_POOL_SIZE` we might
previously check out events from the DB faster than we can service them,
leading to space leaks, weirdness, etc.
Other changes:
- avoid fetch interval sleep latency when we previously did a non-empty
fetch
- prefetch event batch while http pool is working
- warn when it appears we can't keep up with events being generated
- make some effort to process events in creation order so we don't
starve older ones.
ALSO NOTE: HASURA_GRAPHQL_EVENTS_FETCH_INTERVAL changes semantics
slightly, since it only comes into play after an empty fetch. The old
semantics weren't documented in detail, so I think this is fine.
