init: start indexes sync earlier

The mempool load can take a while, and it is not
needed for the indexes' synchronization.

Also, having the mempool load function call
inside 'blockstorage.cpp' wasn't structurally
correct.

src/init.cpp

@@ -1657,7 +1657,10 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     }
 
     chainman.m_load_block = std::thread(&util::TraceThread, "loadblk", [=, &chainman, &args] {
-        ThreadImport(chainman, vImportFiles, ShouldPersistMempool(args) ? MempoolPath(args) : fs::path{});
+        // Import blocks
+        ThreadImport(chainman, vImportFiles);
+        // Load mempool from disk
+        chainman.ActiveChainstate().LoadMempool(ShouldPersistMempool(args) ? MempoolPath(args) : fs::path{});
     });
 
     // Wait for genesis block to be processed