Make broker short-circuit for point queries w/ a limit clause

We've observed `select` and `select distinct` queries with no aggregation and a small limit clause can take a long time to process. We think that the broker is waiting for all results to return before returning to the client, when it can short-circuit and return once enough results to satisfy the limit are available.

cc @Jackie-Jiang

Start at the broker's handling of SELECT and SELECT DISTINCT queries with no aggregation and a small LIMIT clause. Trace how results are collected before being returned to the client, and verify that point queries return once enough results satisfy the limit without changing other query behavior.

Written by the indexing model from the issue text.