Search backend: always stream from evaluate

Created by: camdencheek

This modifies the single place we call evaluate to always pass in a non-nil stream. In order for this behavior to be correct, we need to support a non-nil stream in evaluateAnd and evaluateOr. Currently, streaming is disabled by IsStreamingCompatible whenever the query has an and/or node, but this modifies the logic to only disable streaming when there are multiple basic queries to be run.