MINOR: connection: report the major HTTP version from the MUX for logging (fc_http_major)

MINOR: connection: report the major HTTP version from the MUX for logging (fc_http_major)

A new sample fetch function reports either 1 or 2 for the on-wire encoding,
to indicate if the request was received using the HTTP/1.x format or HTTP/2
format. Note that it reports the on-wire encoding, not the version presented
in the request header.

This will possibly have to evolve if it becomes necessary to report the
encoding on the server side as well.