Forgot to flip the pending boolean back to false, causing it to copy
empty data on the next tick if nothing else triggered a load.
haven't managed to actually reproduce the crash others got, but did
verify that the bad copy was occurring erasing the data.
also fixed a bug with chunk load callback not executing before
another one was scheduled.
No longer clones visible chunks which is causing massive memory
allocation issues, likely the source of Humongous Objects on large servers.
Instead we just synchronize, clear and rebuild, reusing the same object buffers
as before with only 2 small objects created (FastIterator/MapEntry)
This should result in siginificant memory use reduction and improved GC behavior.
