* Add State storage and LastResult argument into Cache so that cache.Types can safely store additional data that is eventually expired.
* New Leaf cache type working and basic tests passing. TODO: more extensive testing for the Root change jitter across blocking requests, test concurrent fetches for different leaves interact nicely with rootsWatcher.
* Add multi-client and delayed rotation tests.
* Typos and cleanup error handling in roots watch
* Add comment about how the FetchResult can be used and change ca leaf state to use a non-pointer state.
* Plumb test override of root CA jitter through TestAgent so that tests are deterministic again!
* Fix failing config test
These were only added as SPIFFE intends to use the in the future but currently does not mandate their usage due to patch support in common TLS implementations and some ambiguity over how to use them with URI SAN certificates. We included them because until now everything seem fine with it, however we've found the latest version of `openssl` (1.1.0h) fails to validate our certificats if its enabled. LibreSSL as installed on OS X by default doesn’t have these issues. For now it's most compatible not to have them and later we can find ways to add constraints with wider compatibility testing.
There are also a lot of small bug fixes found when testing lots of things end-to-end for the first time and some cleanup now it's integrated with real CA code.
Todd Radel