statement-store: process messages from peers in parallel

[alexggh]

Versi testing showed that with 15 peers on_statements ends-up eating a lot of time

polkadot-sdk/substrate/client/network/statement/src/lib.rs

Line 618 in f134881

pub fn on_statements(&mut self, who: PeerId, statements: Statements) {

, because you receive a uniqu estatement multiple times from different peers.

take_recent: 1905.586994 ms
propagate: 15300.300315 ms,
on_stmts: 39057.180931 ms,
submit_worker_0: 27269.046068 ms,
submit_worker_1: 27177.651354 ms,
submit_worker_2: 27178.792487 ms,
submit_worker_3: 27267.589758 ms

Proposed solution

Run the on_statements logic on a separate worker pool rather on the main thread.