Increase benchmark accuracy by using a result based on the last meassured speed after...

Increase benchmark accuracy by using a result based on the last meassured speed after benchmark values changed by less than 0.1% after kernel repeats
Goal is a "what you see is what you get" value compared to: "singlehash -a 3 ?b?b?b?b?b?b?b" -- both with the same fixed -u and -n values
As a positive side-effect, this decreases total benchmark runtime
Add speed_cnt_total and speed_ms_total as a preparation to get rid of SPEED_MAXAGE which produces 0H/s display on very slow-hash types
Replace some floats with double which can (theoretically) become really big