Persist the cache

We persist the guava cache, rather than introducing a new dependency.

This is not ideal; e.g. guava doesn't expose LRU info; doesn't overflow to disk.

We could replace with a 'real' caching system.
Cache the People View

We cache on a per-build basis, the list of all users for that build and all prior builds.

To get the latest info, we walk the build chain building the list of users.  Once we hit
cached info, we merge with the list we've accumulated.