This adds a very cheap connection check to Trilogy. This cheap check can
be used before a connection for example is checked out from a connection
pool.

It allows for making the client more robust in the case of the server
having disconnected (like a failover, an intermediate proxy like
ProxySQL or VTGate restarted etc) but it can be reconnected safely.

It works by doing a non-blocking `recv` to peek if there's any data. If
there is, or if the function would otherwise block it means the
connection is still safe.

If the TCP connection has been closed, `recv` immediately returns with
zero bytes read. This indicates the connection is no longer safe to use.
We do this directly on the file descriptor to be consistent regardless
of things like TLS being used.

Co-authored-by: Daniel Colson <composerinteralia@github.com>
Co-authored-by: John Hawthorn <john@hawthorn.email>