performances: use count(*) instead of count(id) as MySQL can use index keys

src/Storm/Model/PersistenceStrategy/Db.php

@@ -105,7 +105,7 @@ class Storm_Model_PersistenceStrategy_Db extends Storm_Model_PersistenceStrategy
       return 0;
 
     $select->from($this->getTable(),
-                  [sprintf('count(%s) as numberof', $this->_loader->getIdField())]);
+                  [sprintf('count(*) as numberof')]);
     $rows = $this->getTable()->fetchAll($select)->toArray();
     return $rows[0]['numberof'];
   }

tests/Storm/Test/LoaderTest.php

@@ -52,7 +52,7 @@ class Storm_Test_LoaderTest extends Storm_Test_ModelTestCase {
   public function countByWithWhereShouldBuildRightSQL() {
     $select = Storm_Test_ObjectWrapper::mock()
       ->whenCalled('from')
-      ->with($this->_table, array('count(id) as numberof'))
+      ->with($this->_table, ['count(*) as numberof'])
       ->answers(null)
 
       ->whenCalled('where')