MINOR: http-ana: Save the message version in the http_msg structure

MINOR: http-ana: Save the message version in the http_msg structure

When the request or the response is received, the numerical value of the
message version is now saved. To do so, the field "vsn" was added in the
http_msg structure. It is an unsigned char. The 4 MSB bits are used for the
major digit and the 4 LSB bits for the minor one.

Of couse, the version must be valid. the HTX_SL_F_NOT_HTTP flag of the
start-line is used to be sure the version is valid. But because this flag is
quite new, we also take care the string representation of the version is 8
bytes length. 0 means the version is not valid.