This is not specific to authentication, and allows plugins greater flexibility in what events they are interested in. It also adds message handling, and $CONTROL handling.

This allow clients to access their SSL structure and
perform additional verification.

These can be used by plugins to disconnect clients.

This is in preparation for changing to a tree based implementation.

Websockets clients are now part of the by_sock hashtable, so this isn't
needed.

These are for field width and precision for some parameters.

We always get passed the client even if it doesn't have a
username/password now.

If we have e.g. max_inflight_messages set to 1000, and currently have 999 messages inflight, then when we send a new message to a client we have to iterate over the whole list to get to the newest message. This change means that we start of the back of the list to find the newest items, which reduces overhead.

New messages are now queued for clients when old ones are sent, rather than on every iteration of the main loop. This produces good performance improvements.

This should allow tests to be simpler - no more nested ifs.