Follow-up to:
#1908756: Separate Action Links (MENU_LOCAL_ACTION) from hook_menu()
#2031473: Convert menu local actions to plugins so that we can generate dynamic titles and paths
Similar to: #2032303: Cache the result of local tasks
We want to use a persistent cache back-end with local action plugins so discovery is cached and also cache the list of plugins that appears per route to avoid looping over a large list each page view.