Controller Projects::AutocompleteSourcesController#members executes more than 100 SQL queries

See https://gitlab.com/gitlab-org/gitlab-ce/issues/43065#note_153278917 for details about where to start. In particular:

1. The routes N+1 (129 queries, 9%). (Addressed by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26491)
2. The counts N+1 (128 queries, 13%). (Addressed by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26491)
3. The path slowness due to OpenSSL signing (7%). (Addressed by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26858)