Close#2894.
It should be noted that the subscriber's channel is being removed from the
list of receivers and closed, but it is still *in the list of subscribers*
and no unsubscription is performed by WSClient. Which means that RPC server
keeps sending notifications to WSClient and WSClient keeps dropping them
(because there's no receiver for this subscription and it's OK, WSClient
can handle this and this behaviour is documented). However, it's still the
caller's duty to call Unsubscribe() method for this subscription.
Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>
Everywhere including examples, external interop APIs, bindings generators
code and in other valuable places. A couple of `interface{}` usages are
intentionally left in the CHANGELOG.md, documentation and tests.
