`ConnectionHooks` trait for user-provided callbacks on connection hooks

[mempirate]

Context

There is currently no way to track the full lifecycle of individual connections. We should have some way for users to track those, which we can do with "hooks" or "callbacks".

Implementation

We should add a trait ConnectionHooks that a user can implement for their own structs and provide to the sockets. The connection hooks exposes the following methods that will be called at various points in the connection lifecycle:

• on_connection(remote_addr, auth_token): a new connection was created (or recreated). On the server side, this will inject the authentication token (replaces Authenticator trait).
• on_disconnect(remote_addr, error): a connection was disconnected with a transient failure (error & endpoint will be injected). The connection will be retried.
• on_termination(remote_addr, error): a connection encountered a fatal error and will not be retried.

These methods will allow implementers to track the complete lifecycle of any connections.