Clean up sync scheduling (#15043)

It's possible for the scheduler to get in a weird state if the sync
fails while it executes.

This change makes it *only* recreate a job/task if the schedule has
changed or if it is missing. Previously, clearing the state at every
start had bad effects if the JVM had terminated during the sync.

Adds a vector of Exception classes that signal a "fatal" exception
during sync for a specific database. If these exceptions occur, the sync
for that database stops and will pick up next time.

This will have to be expanded per driver, but I don't see a way around
that, as each driver will have its own, unique way of failing.

metabase/metabase#14817