JSON generation on new flakiness dashboard is still too slow

JSON generation on new flakiness dashboard is still too slow
https://bugs.webkit.org/show_bug.cgi?id=123776

Reviewed by Mark Rowe.

* init-database.sql: Added a constraint that result is unique for each (build, test) pair.

* public/include/test-results.php:
(ResultsJSONGenerator::generate): Don't sort results by latest_revision_time since that's done
in the client side anyway.
(update_flakiness_for_build): Don't update is_flaky flag if it's already set to the right value.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@158637 268f45cc-cd09-0410-ab3c-d52691b4dbfc