If `BlockId` implements `Anchor`, the meaning is ambiguous. We cannot
tell whether it means the tx is anchors at the block, or whether it also
means the tx is confirmed at that block.
Instead, `ConfirmationHeightAnchor` and `ConfirmationTimeAnchor` structs
are introduced as non-ambiguous `Anchor` implementations.
Additionally, `TxGraph::relevant_heights` is removed because it is also
ambiguous. What heights are deemed relevant? A simpler and more flexible
method `TxGraph::all_anchors` is introduced instead.
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