kv: only scan separated intents span for QueryResolvedTimestamp requests #69717
Description
Related to #67562 and #67554.
This optimization was originally outlined in the bounded staleness RFC.
Today, a QueryResolvedTimestamp request performs a scan over the MVCC keyspace to retrieve the intents in its target key span.
cockroach/pkg/kv/kvserver/batcheval/cmd_query_resolved_timestamp.go
Lines 92 to 105 in ffbbf81
With intents now separated out into a separate keyspace, we should be able to perform a more efficient scan over the lock table keyspace to retrieve the active intents on the QueryResolvedTimestamp's target key span. This avoids the need to merge the lock table iterator with an MVCC iterator. As a result, we turn an O(num_keys_in_span) operation into an O(num_locks_in_span) operation.
This is a reasonably hard blocker for stage 2 of bounded staleness reads.