RFR: 8135253: Add push method to CollectionSetChooser

[Erik Helin]

Hi all,

this patch adds a new `push` method to CollectionSetChooser so that
regions can be put back at the front of the list. I've split the
patches into three parts to ease reviewing, but I intend to push this as
one commit. While adding the method I also renamed two variables and one
method to make it easier to understand that the CollectionSetChooser
essentially is a stack.

Part 0: Rename _curr_index to _front and _length to _end:
http://cr.openjdk.java.net/~ehelin/8135253/webrev.00-0/

Part 1: Rename remove_and_move_to_next() to pop()
http://cr.openjdk.java.net/~ehelin/8135253/webrev.00-1/

Part 2: Add the push() method
http://cr.openjdk.java.net/~ehelin/8135253/webrev.00-2/

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8135253

Testing:
JPRT

Thanks,
Erik