refactor: Use C++11 range-based for loop

src/wallet/bdb.cpp

@@ -623,8 +623,8 @@ bool BerkeleyDatabase::PeriodicFlush()
     if (!lockDb) return false;
 
     // Don't flush if any databases are in use
-    for (auto it = env->mapFileUseCount.begin() ; it != env->mapFileUseCount.end(); it++) {
+    for (const auto& use_count : env->mapFileUseCount) {
-        if ((*it).second > 0) return false;
+        if (use_count.second > 0) return false;
     }
 
     // Don't flush if there haven't been any batch writes for this database.