init, validation: Fix -reindex option with an existing snapshot

This didn't work for two reasons:
1.) GetSnapshotCoinsDBPath() was used to retrieve the path.
    This requires coins_views to exist, but the initialisation only happens later
    (in CompleteChainstateInitialization) so the node hits an assert in
    CCoinsViewDB& CoinsDB() and crashes.

2.) The snapshot was already activated, so it has the mempool attached.
    Therefore, the mempool needs to be transferred back to the ibd
    chainstate before deleting the snapshot chainstate.

src/validation.cpp

@@ -6140,13 +6140,14 @@ bool ChainstateManager::DeleteSnapshotChainstate()
     Assert(m_snapshot_chainstate);
     Assert(m_ibd_chainstate);
 
-    fs::path snapshot_datadir = GetSnapshotCoinsDBPath(*m_snapshot_chainstate);
+    fs::path snapshot_datadir = Assert(node::FindSnapshotChainstateDir(m_options.datadir)).value();
     if (!DeleteCoinsDBFromDisk(snapshot_datadir, /*is_snapshot=*/ true)) {
         LogPrintf("Deletion of %s failed. Please remove it manually to continue reindexing.\n",
                   fs::PathToString(snapshot_datadir));
         return false;
     }
     m_active_chainstate = m_ibd_chainstate.get();
+    m_active_chainstate->m_mempool = m_snapshot_chainstate->m_mempool;
     m_snapshot_chainstate.reset();
     return true;
 }