Lack of synchronization in DataContext serialization

DataContext is designed to be thread safe, however there is no synchronization in serialization.
This problem shows itself in 4.1.M1 version as ConcurrentModificationException when trying to serialize Context while other threads add or delete objects in it.
In 4.1.M1 underlying data structure was changed, thus revealing this problem. But it also affects earlier versions of Cayenne.