[Echo.Events] several fixes:
 - execute callback for deeepest context even if the context doesn't exists (call it with the empty object to indicate this situation);
 - remove parent context in case it is empty.
[Echo.Events] avoid unnecessary handler deletion
[Echo.Events -> Tests] add several test cases:
 - check subscription structure after event publishing;
 - check that parent context deleted after child unsubscribed if the particular parent context is empty;
 - check that if child's context has the same name as a parent's, this doesn't cause error context creation.\Update test subscription structure.
[Stream control] destroy parent item after children