Generalize single gRPC interface of the storage node to a group of
interfaces. Each interface calls the same RPC handler.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Also use neo-go private key wrapper where possible, as it
already has methods for (un)marshaling.
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
