Support explicit "to" block in game_sendupdates

[domob1812]

Xaya X currently only implements the game_sendupdates RPC method without an explicitly-set "to" block, while Xaya Core allows to specify a desired target block (which will be used instead of the default chain tip if set). libxayagame currently does not require this, so it was not implemented in Xaya X for simplicity.

However, it could be useful to support this, at least with the "to" block set to an old mainchain block. With this, it would be possible to roll back GSPs on-demand to an old block, e.g. for debugging purposes. Implementing this only for mainchain blocks and not (known) fork blocks is probably all that is needed, and should be relatively straight forward to do.