[net processing] Guard m_continuation_block with m_block_inv_mutex

Summary:
Completes backport of core#19829:
https://github.com/bitcoin/bitcoin/pull/19829/commits/3002b4af2b4fde63026f8f7c575452a5c989c662

Depends on D10871.

Ref T1696.

Test Plan:
With Clang and debug:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10872