[chain_redesign] BlockId should not implement Anchor

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.

crates/chain/src/tx_graph.rs

@@ -349,6 +349,11 @@ impl<A> TxGraph<A> {
             .filter(move |(_, conflicting_txid)| *conflicting_txid != txid)
     }
 
+    /// Get all transaction anchors known by [`TxGraph`].
+    pub fn all_anchors(&self) -> &BTreeSet<(A, Txid)> {
+        &self.anchors
+    }
+
     /// Whether the graph has any transactions or outputs in it.
     pub fn is_empty(&self) -> bool {
         self.txs.is_empty()
@@ -592,21 +597,6 @@ impl<A: Clone + Ord> TxGraph<A> {
 }
 
 impl<A: Anchor> TxGraph<A> {
-    /// Get all heights that are relevant to the graph.
-    pub fn relevant_heights(&self) -> impl Iterator<Item = u32> + '_ {
-        let mut last_height = Option::<u32>::None;
-        self.anchors
-            .iter()
-            .map(|(a, _)| a.anchor_block().height)
-            .filter(move |&height| {
-                let is_unique = Some(height) != last_height;
-                if is_unique {
-                    last_height = Some(height);
-                }
-                is_unique
-            })
-    }
-
     /// Get the position of the transaction in `chain` with tip `chain_tip`.
     ///
     /// If the given transaction of `txid` does not exist in the chain of `chain_tip`, `None` is