search: parallelize repositories resolver gitserver requests
Created by: kzh
This PR aims to improve the speed of the repositories resolver to accompany the addition of the repohascommitafter
filter. It switches to concurrent gitserver requests.
The current repohascommitafter
implementation (synchronous) times out (when searching over all repos) on the large test sourcegraph instance. This PR should provide a fix for that issue.
Fixes #4567