improve sql parser performance

src/main/java/com/alibaba/druid/util/StringUtils.java

@@ -91,4 +91,18 @@ public class StringUtils {
 
         return false;
     }
+    
+    public static int lowerHashCode(String text) {
+//        return text.toLowerCase().hashCode();
+        int h = 0;
+        for (int i = 0; i < text.length(); ++i) {
+            char ch = text.charAt(i);
+            if (ch >= 'A' && ch <= 'Z') {
+                ch = (char) (ch + 32);
+            }
+
+            h = 31 * h + ch;
+        }
+        return h;
+    }
 }