RFR (S): Record accurate partial GC timings

[Aleksey Shipilev]

Hi,

We now record only the crude partial GC timing, which misses TTSP and
preparation times. Also it is not counted towards total pause times.

Fix:
 http://cr.openjdk.java.net/~shade/shenandoah/partial-stats/webrev.02/

Sample output:
 https://paste.fedoraproject.org/paste/G7KiyaWaqEsDp9TkKosxgV5M1UNdIGYhyRLivL9gydE=

Thanks,
-Aleksey