[bdk_chain_redesign] chain_oracle::Cache

Introduce `chain_oracle::Cache` which is a cache for requests to the
chain oracle. `ChainOracle` has also been moved to the `chain_oracle`
module.

Introduce `get_tip_in_best_chain` method to the `ChainOracle` trait.
This allows for guaranteeing that chain state can be consistent across
operations with `IndexedTxGraph`.

crates/chain/src/tx_graph.rs

@@ -586,11 +586,12 @@ impl<A: Clone + Ord> TxGraph<A> {
 
 impl<A: BlockAnchor> TxGraph<A> {
     /// Get all heights that are relevant to the graph.
-    pub fn relevant_heights(&self) -> BTreeSet<u32> {
+    pub fn relevant_heights(&self) -> impl Iterator<Item = u32> + '_ {
+        let mut visited = HashSet::new();
         self.anchors
             .iter()
             .map(|(a, _)| a.anchor_block().height)
-            .collect()
+            .filter(move |&h| visited.insert(h))
     }
 
     /// Determines whether a transaction of `txid` is in the best chain.