Search backend: DRY out searcher streams

Created by: camdencheek

We currently have two different stream limiters in searcher: one for streaming and one for batch. I honestly don't remember why, but we can implement the collecting stream by appending in the callback. Just trying to reduce some maintenance costs here since I've had to change both of these in multiple times during these multiline refactors.

Stacked on https://github.com/sourcegraph/sourcegraph/pull/36431

Test plan

Many tests depend on this batch limit. Its behavior should be well-covered.