Perf-o-matic should memcache dashboard images

Perf-o-matic should memcache dashboard images
https://bugs.webkit.org/show_bug.cgi?id=80349

Reviewed by Eric Seidel.

Added DashboardImage.create and DashboardImage.get_image to encapsulate memcache.
Also replaced transaction in DashboardImage.set_cache by a single put since it duplicates
what put does by default.

Also removed redundant cache_* functions and merged them into handler code.

* Websites/webkit-perf.appspot.com/controller.py:
(ManifestUpdateHandler.post):
(CachedManifestHandler.get):
(DashboardUpdateHandler.post):
(CachedDashboardHandler.get):
(RunsUpdateHandler):
(RunsUpdateHandler.post):
(RunsChartHandler):
(RunsChartHandler.post):
(DashboardImageHandler.get):
* Websites/webkit-perf.appspot.com/models.py:
(PersistentCache.set_cache):
(DashboardImage):
(DashboardImage.create):
(DashboardImage.get_image):
* Websites/webkit-perf.appspot.com/models_unittest.py:
(PersistentCacheTests.setUp):
(PersistentCacheTests.test_set_cache):
(PersistentCacheTests.test_get_cache):
(DashboardImageTests.setUp):
(DashboardImageTests):
(DashboardImageTests.test_create):
(DashboardImageTests.test_get):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@109821 268f45cc-cd09-0410-ab3c-d52691b4dbfc