Enable memcache on webkit-perf.appspot.com

Enable memcache on webkit-perf.appspot.com
https://bugs.webkit.org/show_bug.cgi?id=77378

Reviewed by Adam Barth.

Eanble memcache on dashboard, manifest, and runs handlers. Clear appropriate caches when new runs are reported,
or new models are created. We flush all caches when tests are merged since it's hard to figure out dependencies
in that case. Luckily, we merge tests only occassionally and manually (or hope so) so this shouldn't be an issue.

* Websites/webkit-perf.appspot.com/create_handler.py:
(CreateHandler.post):
* Websites/webkit-perf.appspot.com/dashboard_handler.py:
(DashboardHandler.get):
* Websites/webkit-perf.appspot.com/manifest_handler.py:
(ManifestHandler.get):
* Websites/webkit-perf.appspot.com/merge_tests_handler.py:
(MergeTestsHandler.post):
* Websites/webkit-perf.appspot.com/models.py:
(createInTransactionWithNumericIdHolder):
(Test):
(Test.cacheKey):
* Websites/webkit-perf.appspot.com/report_handler.py:
(ReportHandler.post):
(ReportHandler._addTestIfNeeded):
* Websites/webkit-perf.appspot.com/runs_handler.py:
(RunsHandler.get):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106308 268f45cc-cd09-0410-ab3c-d52691b4dbfc