Fix the async flow with the Router

It had sequential async calls with complexity O(n), with a variety of redundant
calls. There was also a constant of... 4? 5? for each item. Now, the total
sequence depth is just 3-4.

Cargo.lock

@@ -9446,8 +9446,8 @@ dependencies = [
  "alloy-sol-macro",
  "alloy-sol-types",
  "alloy-transport",
+ "futures-util",
  "serai-processor-ethereum-primitives",
- "tokio",
 ]
 
 [[package]]
@@ -9480,6 +9480,7 @@ dependencies = [
  "borsh",
  "build-solidity-contracts",
  "ethereum-schnorr-contract",
+ "futures-util",
  "group",
  "k256",
  "rand_core",