[net processing] Only send a getheaders for one block in an INV

Summary:
Headers-first is the primary method of announcement on the network. If a
node fell back sending blocks by inv, it's probably for a re-org. The
final block hash provided should be the highest, so send a getheaders
and then fetch the blocks we need to catch up.

This is a backport of Core PR18962

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9113