Missing doc change states on parallel docs save

[KholmatovS]

There's bug that snapshot's docChanges state doesn't contain all information when docs are saved in parallel:
Example:

    final op1 = db.collection('docs').doc('1').set(<String, dynamic>{
      'name': 'Norman',
      'age': 28,
    });

    final op2 = db.collection('docs').doc('2').set(<String, dynamic>{
      'name': 'Peter',
      'age': 19,
    });

    /// Parallel save
    await Future.wait([op1, op2]);

If we query docs with where('age', isLessThan: 30) before saving the above docs, only single added doc change event will be emitted. However, expected will be two, per each snapshot data

Please refer to the test file in the PR for the details of failing test case
PR: #312