[net processing] Move nStartingHeight to Peer

Summary:
Partial backport of core#19829:
https://github.com/bitcoin/bitcoin/pull/19829/commits/77a2c2f8f91a5c5a140fd970f9a3a142b43902bf

Ref T1696.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10866