change how tracing query comments are handled

[systay]

When sending the trace information over the mysql protocol, we need to be able to encode a string-string map into a string that can be put into a query as a comment. The current way of doing it is broken, since it is not protecting the map serialisation from clashing with SQL.

This PR changes the approach to use first JSON as a way of encoding the map, and then base64 encoding to make sure that the json doesn't clash with any SQL syntax rules.

On the client side, one would need to pick up the current tracing information from openTracing, encode it as JSON, base64 encode the json-string, and stick this into the comment. Example:

openTracingData := map[string]string{
	"uber-trace-id":"123:456:789:1",
	"other data":   "something else",
}

jsonBytes, _ := json.Marshal(openTracingData)
encodedComment := base64.StdEncoding.EncodeToString(jsonBytes)

originalQuery := "select 42"

fullQuery := fmt.Sprintf("/*VT_SPAN_CONTEXT=%s*/ %s", encodedComment, originalQuery)

fmt.Println(fullQuery)

Result:

/*VT_SPAN_CONTEXT=eyJvdGhlciBkYXRhIjoic29tZXRoaW5nIGVsc2UiLCJ1YmVyLXRyYWNlLWlkIjoiMTIzOjQ1Njo3ODk6MSJ9*/ select 42

This PR replaces #6387