Add instant, obligatory, server-initiated failover mechanism to MariaDB Client/Server Protocol

See this comment on MDEV-15935, where otto explains how such a mechanism is a very simple and useful one for redirecting new client connections, and extremely time-tested in other application protocols, particularly HTTP.

An alternative solution of "advisory" server-requested redirection/failover was chosen for MDEV-15935.

As I wrote in this comment:

As you [serg] noted yourself on the mailing list, *you don't know of any actual use cases for advisory-only redirection*. By contrast, we are aware of one major use case for "forceful" redirection, graceful server shutdown, as discussed here and on the mailing list."