G1: Rename measured pause time ratios

G1 currently uses two values to drive heap resizing (expansion):

- recent_avg_pause_time_ratio: contains the (calculated) pause time ratio over the last X mutator/gc phases
- last_pause_time_ratio: contains the (calculated) pause time ratio over the last(*) mutator/gc phase

(*) this is not entirely true, see JDK-8243672.

I suggest to rename them to long/short_term_pause_time_ratio to be a bit more flexible about the contents.