Process block cleanup first. Fixes BUKKIT-4103

When setting a block the server sets the new block ID, calls the cleanup
on the old block, then calls the placement logic for the new block. This
would normally work correctly but we have logic to prevent errors that is
causing one here. When trying to clean up the old block we notice the block
ID has changed and try to fix things, this causes the cleanup logic to
fail. To ensure cleanup happens correctly we now do all the cleanup before
changing the block which avoids triggering the fixup code.