Add examples for nested sagas

[andreyleskov]

It is a common scenario when one saga initiate other saga and need to wait for its end.
We can correlate this sagas by one Id, but in this case we can face events or persistence id overlapping.
Need to create how-to example and test to demonstrate how to start parent saga with child saga having different Ids.

[andreyleskov]

This case should be automatically covered by the correlation and causation id mechanism, but we still need an example