search: report function for aggregator and one mutex

Created by: keegancsmith

This PR contains two commits which do some refactoring to the recently introduced aggregator struct. The first is the introduction of a helper function to report "simple" results. We still need special support for file matches, and this will be addressed in another PR since that may be more controversial.

This PR also removes the many mutexes, and replaces it with one. This makes it easier to reason about the critical sections, as well as allows future changes to this code to be clearer.