fix: #900 replace callback implementation by the promise one

Signed-off-by: Vincent Vandenschrick <vvandens@gmail.com>

Author: vvandens

lib/mongodb.js

@@ -2349,23 +2349,17 @@ MongoDB.prototype.beginTransaction = function(isolationLevel, cb) {
 };
 
 MongoDB.prototype.commit = function(tx, cb) {
-  tx.commitTransaction(function(err) {
-    tx.endSession(null, function(error) {
-      if (err) return cb(err);
-      if (error) return cb(error);
-      cb();
-    });
-  });
+  tx.commitTransaction()
+    .then(() => tx.endSession())
+    .then(() => cb())
+    .catch((err) => cb(err));
 };
 
 MongoDB.prototype.rollback = function(tx, cb) {
-  tx.abortTransaction(function(err) {
-    tx.endSession(null, function(error) {
-      if (err) return cb(err);
-      if (error) return cb(error);
-      cb();
-    });
-  });
+  tx.abortTransaction()
+    .then(() => tx.endSession())
+    .then(() => cb())
+    .catch((err) => cb(err));
 };
 
 function isInTransation(options) {