World Rewrite -- World Lock

Given that I'm now using a sensible memory layout for the blocks in a
column, rather than trying to emulate Mojang's wire layout in memory, I
can switch back to my old, granular locking idea.

Accordingly I've reimplemented the WorldLock class, which provides
granular, fair locking on individual blocks, individual columns, or the
entire world.

I'm planning on upgrading it to support upgrading locks, so that a lock
holder can add blocks/columns/etc. to a held lock, enabling them to lock
a minimal amount first, check, and then proceed to lock more.

Made a few other relatively minor changes, such as phasing out the
WorldGeneratorContainer class, and changing some inconsistent naming.
World Lock Bugfix

Fixed a bug which was causing callbacks not to be properly loaded/fired
when upgrading a lock.
World Lock -- Upgradeability

WorldLock locks can now be upgraded.