RFR (S): Pre-touch bitmaps

[Aleksey Shipilev]

Hi,

Turns out, pre-touching bitmaps helps for the same reason pre-touching regions
helps. So, we can touch the bitmaps when touching the regions too:
  http://cr.openjdk.java.net/~shade/shenandoah/bitmaps-pretouch/webrev.01/

Previous incarnation of the patch improved reset times:

  baseline: Reset Bitmaps = 5.52 s (a =   204332 us) (n =    27)
             (lvls, us =     7402,   197266,   214844,   232422,   258593)

pretouched: Reset Bitmaps = 2.34 s (a =    86509 us) (n =    27)
             (lvls, us =     1113,    81641,    88086,   101562,   117187)

I will retest this on the same machine once it is free.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey