HTTP/2: invalid connection preface logging (ticket #1981).

HTTP/2: invalid connection preface logging (ticket #1981).

Previously, invalid connection preface errors were only logged at debug
level, providing no visible feedback, in particular, when a plain text
HTTP/2 listening socket is erroneously used for HTTP/1.x connections.
Now these are explicitly logged at the info level, much like other
client-related errors.