RFR (M): 8151126: Clean up duplicate code for clearing the mark bitmaps

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that removes duplicate code for
clearing the next mark bitmap. This duplication is in
 G1CMBitMap::clearAll() and G1ConcurrentMark::clearNextBitmap() for
clearing the "next" bitmap which has been moved into
 G1ConcurrentMark::clear_bitmap().

Most changes are moving the AbstractGangTask that is used for this
around.

There is JDK-8151171 to further clean up naming in the g1ConcurrentMark
files.

CR:
https://bugs.openjdk.java.net/browse/JDK-8151171
Webrev:
http://cr.openjdk.java.net/~tschatzl/8151126/webrev/
Testing:
jprt, vm.gc testlist

Thanks,
  Thomas