This makes it easier for downstream projects (forks) to replace the
version fetching system with their own. It is as simple as implementing
an interface and overriding the default implementation of
org.bukkit.UnsafeValues#getVersionFetcher()
It also makes it easier for us to organize things like the version
history feature.
Lastly I have updated the paper implementation to check against the site
API rather than against jenkins.
We were missing a synchronize on a get if cached method, however
it appears that using a ConcurrentHashMap is a better solution
so readers can avoid locking if they just want a cached value.
Existing synchronization for writers remains untouched, the
ConcurrentHashMap is just so readers can safely read without
synchronization
Zach Brown
Spottedleaf
Shane Freeder