Rename vBlockHashesToAnnounce and vInventoryBlockToSend

Summary:
Original rename script:

-BEGIN VERIFY SCRIPT-
sed -i 's/vBlockHashesToAnnounce/m_blocks_for_headers_relay/g' src/net_processing.*
sed -i 's/vInventoryBlockToSend/m_blocks_for_inv_relay/g' src/net_processing.*
-END VERIFY SCRIPT-

Partial backport of core#19829:
https://github.com/bitcoin/bitcoin/pull/19829/commits/c853ef002ee7074b6e20eb8f58138c8293846424

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/D10870