Delete all ancestors of the tree root after playing a move

[tommadams]

That way the tree's "root" is actually the real root of the tree.
Also the current implementation of the undo GTP command is completely broken I think because it just changes the root to be its own parent but the parent's visit counts no longer match the counts of its children (because they get pruned).

This would require updating at least GtpClient::HandleUndo and MiniguiGtpClient::ReplaySgf.

[tommadams]

The other really important thing is to update the root node's superko_cache.