use cached most recent compact block when available

Summary:
We lost the m_most_recent_compact_block optimization when removing segwit, because in Core's codebase it used to be gated by state.fWantsCmpctWitness. See rABCf7a1e4adc8a9 and D2620.
As a result m_most_recent_compact_block is computed but unused. Make use of it.

This brings our codebase in line with Bitcoin Core's codebase just before core#24062 (next backport)

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14878