Enforce that ProtocolId and the notifications protocol names should be UTF-8

[tomaka]

The ProtocolId (ksmcc, dot, etc.) is, in practice, always a string. But at some point within the code we convert it into bytes, then employ shenanigans to turn it back into a string.

We should entirely remove maybe_utf8_bytes_to_string and enforce that ProtocolId is a string.

[tomaka]

I've re-purposes this issue to cover both ProtocolId and the notification protocols names.