Correct internal node children on reorg.

Previously the code was only adding a new block node as a child in the
inernal node index for the cases it extended the main chain or a side
chain and not for a node which caused a reorg.  This resulted in the block
node pruning code not clearing the parent link of reorged nodes which
ultimately led to a sanity check error accordingly.

This commit resolves the issue by ensuring new block nodes are added as
children of their respective parents in all cases.

Closes #4.