Fix O(n²) find_head and stack overflow in filter_block_tree (#9090)

Co-Authored-By: dapplion <35266934+dapplion@users.noreply.github.com>

Co-Authored-By: Michael Sproul <michaelsproul@users.noreply.github.com>

consensus/proto_array/src/proto_array_fork_choice.rs

@@ -514,6 +514,7 @@ impl ProtoArrayForkChoice {
             prune_threshold: DEFAULT_PRUNE_THRESHOLD,
             nodes: Vec::with_capacity(1),
             indices: HashMap::with_capacity(1),
+            children: Vec::with_capacity(1),
         };
 
         let block = Block {