RFR (T): 8222105: Rename G1Policy::adaptive_young_list_length

[Thomas Schatzl]

Hi all,

  in the review for 8218668: Clean up evacuation of optional collection
set Kim noted that the name for G1Policy::adaptive_young_list_length
should be improved by adding a "use_" prefix to it.

This change does exactly that.

CR:
https://bugs.openjdk.java.net/browse/JDK-8222105
Webrev:
http://cr.openjdk.java.net/~tschatzl/8222105/webrev/
Testing:
local compilation

Thanks,
  Thomas