use StorageAdapter#size to reduce IO

package.json

@@ -43,6 +43,10 @@
     {
       "name": "Gordon Hall",
       "email": "gordon@storj.io"
+    },
+    {
+      "name": "littleskunk",
+      "url": "https://github.com/littleskunk"
     }
   ],
   "license": "AGPL-3.0",

script/farmer.js

@@ -45,7 +45,7 @@ function sendFarmerState() {
 }
 
 function updatePercentUsed() {
-  utils.getDirectorySize(config.storagePath, (err, result) => {
+  config.storageManager._storage.size((err, result) => {
     if (result) {
       farmerState.spaceUsed = bytes(result);
       farmerState.percentUsed = ((result / spaceAllocation) * 100).toFixed();