[electrum] avoid unneccessary bytes <-> hex conversion for block header

Summary:
Make serialize_header return bytes to avoid converting to bytes at all callsites.

Depends on D14569

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14570