Fail connection in receipt of unsubscribed message.

[Eric Voskuil]

Given the current design:

• Protocols should register for all message types that the session will solicit or that may be sent by peers unsolicited.
• The start handler should block until initial protocols are registered (see related issue).
• Any post-handshake solicitation should register subscription prior to sending the solicitation and deregister if and when the solicitation is expired.

any message that is received without a registered handler is in violation of the session's intended protocol and should result in channel stop. Otherwise DOS based on sending large unhandled messages is unchecked.

A session could devise other DOS protections, such as channel lifetime or byte limits so this feature should be enabled at proxy construct.

[Eric Voskuil]

This is complicated by the fact that subscribers are removed from the subscription list during processing.