Make target block request required ZMQ notifications

[domob1812]

When a target block is set (see #129) in Game, the game_sendupdates requests should set the target block as explicit to block; right now, no to is set, so that notifications are always requested to the chain tip (and that won't help to sync to a target that is in the past).

Note that this requires Xaya X to implement a to block first (see xaya/xayax#7).