The previous fix for the ongoing event fetches counter
(8eec25a1d9) was both insufficient and
incorrect.
When the database is unreachable, `_do_fetch` never gets run and so
`_event_fetch_ongoing` is never decremented.
The previous fix also moved the `_event_fetch_ongoing` decrement outside
of the `_event_fetch_lock` which allowed race conditions to corrupt the
counter.
