p2p: Serialize cmpctblock at most once in NewPoWValidBlock

Summary:
Instead of serializing for each peer, serialize at most once and copy the raw data for each peer.

This is a backport of core#23880

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14774