p2p: For assumeutxo, download snapshot chain before background chain

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.
Only existing peers are affected by this bug.

src/net_processing.cpp

@@ -1397,9 +1397,12 @@ void PeerManagerImpl::FindNextBlocksToDownload(const Peer& peer, unsigned int co
         return;
     }
 
-    if (state->pindexLastCommonBlock == nullptr) {
+    std::optional<int> snapshot_height{m_chainman.GetSnapshotBaseHeight()};
+    if (state->pindexLastCommonBlock == nullptr ||
+        (snapshot_height.has_value() && state->pindexLastCommonBlock->nHeight < snapshot_height.value())) {
         // Bootstrap quickly by guessing a parent of our best tip is the forking point.
         // Guessing wrong in either direction is not a problem.
+        // Also applies after loading a snapshot, when we want to prioritise loading the snapshot chain and therefore need to skip ahead.
         state->pindexLastCommonBlock = m_chainman.ActiveChain()[std::min(state->pindexBestKnownBlock->nHeight, m_chainman.ActiveChain().Height())];
     }