Writes the outer snapshot, i.e. any information beyond the nested serializers of the outer serializer.
The base implementation of this methods writes nothing, i.e. it assumes that the outer serializer
only has nested serializers and no extra information. Otherwise, if the outer serializer contains
some extra information that needs to be persisted as part of the serializer snapshot, this
must be overridden. Note that this method and the corresponding methods
#readOuterSnapshot(int,DataInputView,ClassLoader),
#isOuterSnapshotCompatible(TypeSerializer)needs to be implemented.