p2p: For assumeutxo, download snapshot chain before background chain

Summary:
After loading a snapshot, pindexLastCommonBlock is usually already set
to some block for existing peers. That means we'd continue syncing the
background chain from those peers instead of prioritising the snapshot
chain, which defeats the purpose of doing assumeutxo in the first place.

This is a backport of core#29519
https://github.com/bitcoin/bitcoin/pull/29519/commits/49d569cb1fdd62a9da8dff51dccaf4680fe3d0eb
Depends on D18455

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18456