Verify server ID before `KILL QUERY`

[bgrainger]

MySqlCommand.Cancel attempts to ensure that KILL QUERY runs on the correct server by connecting directly to its IP address:

MySqlConnector/src/MySqlConnector/MySqlConnection.cs

Lines 913 to 921 in 4cbcfc7

	// connect directly to the session's IP address to ensure we're cancelling the query on the right server (in a load-balanced scenario)
	IPAddress? ipAddress = null;
	if (session.IPEndPoint is { Address: { } sessionIpAddress, Port: { } port })
	{
	// set the hostname to the existing session's hostname (for SSL validation)
	csb.Server = session.HostName;
	csb.Port = (uint) port;
	ipAddress = sessionIpAddress;
	}

In a layer 4 load balancing scenario, this might not be sufficient. Running SELECT @@server_uuid; or SELECT @@server_id; at connection establishment time (depending on which servers support those system variables) could provide a unique ID that the Cancel implementation could check before it cancels the query. This would avoid cancelling a query that just happens to use the same server thread ID on a different server.

[bgrainger]

SELECT @@hostname; might be a good fallback if server UUID isn't available.

[bgrainger]

Server Support

Server	Hostname	Server UUID
MySQL 5.5	✅	❌
MySQL 5.6	✅	✅
MySQL 5.7+	✅	✅
MariaDB 11.0	✅	❌
PlanetScale	?	?
ProxySQL	✅
SingleStore	✅	❌
TiDB	?	?