Add cache for parallel HTTP requests (#4879)

Cargo.lock

@@ -569,9 +569,9 @@ dependencies = [
  "lru",
  "maplit",
  "merkle_proof",
- "oneshot_broadcast",
  "operation_pool",
  "parking_lot 0.12.1",
+ "promise_cache",
  "proto_array",
  "rand 0.8.5",
  "rayon",
@@ -6160,6 +6160,16 @@ dependencies = [
  "syn 2.0.48",
 ]
 
+[[package]]
+name = "promise_cache"
+version = "0.1.0"
+dependencies = [
+ "derivative",
+ "itertools",
+ "oneshot_broadcast",
+ "slog",
+]
+
 [[package]]
 name = "proto_array"
 version = "0.2.0"