MINOR: stats: add server revision id support

MINOR: stats: add server revision id support

Make use of the new srv->rid value in stats.

Stat is referred as ST_F_SRID, it is now used in stats_fill_sv_stats
function in order to be included in csv and json stats dumps.

Moreover, "rid: $value" will be displayed next to server puid
in html stats page if "stats show-legend" is specified in the stats frontend.
(mouse hovering tooltip)

Depends on the following commit:
	"MINOR: server: add srv->rid (revision id) value"