Fix missed SwitchOnFirst drain and inner stream cancel (#2076)

This commit fixes missed cases where a drain is necessary.
It also adds Inner stream erroring if it appeared after the
parent cancellation happened (to make sure disconnected inner
stream will not be hanged for forever).
It includes refactoring to reduce code duplication.

Reviewed-in: #2076
Signed-off-by: Oleh Dokuka <shadowgun@i.ua>