Cache lazy-evaluated field

Reading the data from the buffer repeatedly can be quite slow. Caching
this data can make it to be significantly faster.

Sequelize Before Change
Call to doSomething took 2571.7481999993324 milliseconds

Sequelize After Change
Call to doSomething took 1230.2330999970436 milliseconds

Author: testn

lib/packets/column_definition.js

@@ -112,10 +112,19 @@ const addString = function(name) {
     get: function() {
       const start = this[`_${name}Start`];
       const end = start + this[`_${name}Length`];
-      return StringParser.decode(
+      const val = StringParser.decode(
         this._buf.slice(start, end),
         this.encoding === 'binary' ? this._clientEncoding : this.encoding
       );
+      
+      Object.defineProperty(this, name, {
+        value: val,
+        writable: false,
+        configurable: false,
+        enumerable: false
+      });
+
+      return val;
     }
   });
 };