Improve search_events view definition

[LukasKalbertodt]

Our current definition of the search_events view is not optimal in regards to performance. We have to use cases:

• Fetching only a few specific events (e.g. done by tobira search-index update)
• Fetching all events (e.g. done by tobira search-index rebuild)

The current definition is fast for fetching all events, but slow for only fetching a few. Well, it takes the exact same time in both cases, but that's the point: the "only few" should be much faster, scaling with the number of events we are looking up. That's because the current view definition always does a full scan of realms (due to the join).

An alternative method would be to have a nested query in the select as host_realms instead of the join. That's how search_series does it. Thats way faster for "only a few events" (scales appropriately), but is notably slower for "all events". It's also questionable whether it's maybe notably slower for search_series too, but we haven't noticed it bc there are fewer series?

In any case, it would be nice to find a query that combines the best of both worlds.