Unsafe search response DB operations

Sentry reported an IndexError in search.response.event_datatables_response here.

Looking into it, I've realized that this call to count() performs a SQL SELECT COUNT(*), which can then be outdated by the time we iterate over a second query. This all seems to be part of optimizations made in !163 (merged). I would consider reverting that MR, or if avoiding list.append is actually having a measurable performance benefit, using something like a pre-allocated 2D buffer array.