Check pending blocks before falling back to world. Fixes BUKKIT-5122

When growing trees we use a BlockChangeDelegate which queues up the block
changes so plugins can modify/block/log tree growing. However, we always
check the actual world when checking for existing blocks. This means when
the tree growing code checks to see if putting a leaf in a block is valid
it may incorrectly overwrite a log block that should exist in that
location. To ensure trees grow correctly we now check the delegate itself
for blocks that match the queried location before checking the world.