search: shortcircuit on count for search expressions

Created by: rvantonder

After merging #13907, my nice big queries for blog post became slow, because no shortcircuiting was happening (it's as if count:30 was specified for each or subexpression, and always run). This PR:

• shortcircuits on subexpression results once we've hit 'enough' results (30 by default).
• additionally, #13907 invalidates resolved repos for every or subexpression. This is overkill: we only need to invalidate the repo cache if there are more than one repo or repogroup field.

For now it just matters that the search integration tests are green, so that I can publish the blog post: https://buildkite.com/sourcegraph/sourcegraph/builds/76866. I will follow up with an integration test that checks shortcircuiting is honored as expected--this needs some more significant changes to gqltest.