[net processing] Move hashContinue to net processing

Summary:

Also rename to m_continuation_block to better communicate meaning.

Partial backport of core#19829:
https://github.com/bitcoin/bitcoin/pull/19829/commits/184557e8e03f76ff18dacdb32c12692d8578691f

Depends on D10870.

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/D10871