Drop indices concurrently on background updates (#18091)

Otherwise these can race with other long running queries and lock out
all other queries.

This caused problems in v1.22.0 as we added an index to `events` table
in #17948, but that got interrupted and so next time we ran the
background update we needed to delete the half-finished index. However,
that got blocked behind some long running queries and then locked other
queries out (stopping workers from even starting).