LoadMempool: Pass in load_path, stop using gArgs

Also:
1. Have CChainState::LoadMempool and ::ThreadImport take in paths and
   pass it through untouched to LoadMempool.
2. Make LoadMempool exit early if the load_path is empty.
3. Adjust the call to ::ThreadImport in ::AppInitMain to correctly pass
   in an empty path if mempool persistence is disabled.

src/init.cpp

@@ -1673,7 +1673,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     }
 
     chainman.m_load_block = std::thread(&util::TraceThread, "loadblk", [=, &chainman, &args] {
-        ThreadImport(chainman, vImportFiles, args);
+        ThreadImport(chainman, vImportFiles, args, ShouldPersistMempool(args) ? MempoolPath(args) : fs::path{});
     });
 
     // Wait for genesis block to be processed