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 [[bitcoin/bitcoin#29519 | core#29519]]
bitcoin/bitcoin@49d569c
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

Author: mzumsande

src/net_processing.cpp

@@ -2025,9 +2025,13 @@ void PeerManagerImpl::FindNextBlocksToDownload(
         return;
     }
 
-    if (state->pindexLastCommonBlock == nullptr) {
-        // Bootstrap quickly by guessing a parent of our best tip is the forking
-        // point. Guessing wrong in either direction is not a problem.
+    // Bootstrap quickly by guessing a parent of our best tip is the forking
+    // point. Guessing wrong in either direction is not a problem. Also reset
+    // pindexLastCommonBlock after a snapshot was loaded, so that blocks after
+    // the snapshot will be prioritised for download.
+    if (state->pindexLastCommonBlock == nullptr ||
+        (snap_base &&
+         state->pindexLastCommonBlock->nHeight < snap_base->nHeight)) {
         state->pindexLastCommonBlock =
             m_chainman
                 .ActiveChain()[std::min(state->pindexBestKnownBlock->nHeight,