Fixed Eager Dimension Data Creation

1. Fixed a design flaw in PocketManager. We originally assumed that all
requests to PocketManager.getDimensionData() had to be legitimate
requests for dimensions that existed. That was true in most cases, but
for things like processing user commands, it was dangerously optimistic.
It was possible that a flaw in DD's usage of that function could be
exploited by a player to trick the mod into pre-registering dimension
data for a non-existent dimension. That would declare the dimension as a
root. DD would crash later if Forge ever allocated that ID for a pocket
dimension. The new implementation is almost the same as the old one, but
allows us to differentiate between cases when we can eagerly create
dimension data, and cases in which the absence of a dimension should
cause a crash to alert us of a design flaw.
2. Remove the pocket regeneration code from PocketBuilder. We simply
don't support pocket regeneration and it's unlikely it'll ever be
implemented because it's a difficult issue. Wiping out pockets
completely is easier. We can always recover the code from this commit if
it's needed later.
3. Minor changes: removed some debug prints from PocketManager and
changed some static accesses in PocketBuilder.