Cache-control should be set only on api/runs

Cache-control should be set only on api/runs
https://bugs.webkit.org/show_bug.cgi?id=140312

Reviewed by Andreas Kling.

Some JSON APIs such as api/analysis-tasks can't be cached even for a short period of time (e.g. a few minutes)
since they can be modified by the user on demand. Since only api/runs.php takes a long time to generate JSONs,
just set cache-control there instead of json-header.php which is used by other JSON APIs.

Also set date_default_timezone_set in db.php since we never use non-UTC timezone in our scripts.

* public/api/analysis-tasks.php:
* public/api/runs.php: Set the cache control headers.
* public/api/test-groups.php:
* public/include/db.php: Set the default timezone to UTC.
* public/include/json-header.php: Don't set the cache control headers.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@178208 268f45cc-cd09-0410-ab3c-d52691b4dbfc