Add a release note for the parralel compact block download

Summary: This is actually a fix for a bug that could delay the download of a block if the peer is stalled.

Test Plan: Read it.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17085