Add per-host reverse index for small-target live queries

Resolves #42441

Store queries that target at most
redis.live_query_small_target_threshold hosts (default 1000) in a
per-host reverse index instead instead of a per-query bitfield indexed
by host ID.

Setting the threshold to 0 disables the reverse index (no query has <= 0
targets), serving as the kill-switch.

changes/42441-live-query-redis-scaling

@@ -0,0 +1 @@
+- Fixed a bug where running many concurrent live queries that each target a small number of hosts could overload Redis and slow down host check-ins.