RFR (S): 7127700: G1: Replace G1PLAB with PLAB

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that removes G1PLAB and uses
the PLAB class throughout?

Although the CR indicates that the remaining _retired member of G1PLAB
might be useful, I think the additional check it provides is not worth
the effort, so I removed it.

CR:
https://bugs.openjdk.java.net/browse/JDK-7127700
Webrev:
http://cr.openjdk.java.net/~tschatzl/7127700/webrev/
Testing:
hs-tier 1-5

P.S: there is an alternate webrev at http://cr.openjdk.java.net/~tschat
zl/7127700/webrev.alt that keeps the _retired flag and moves it to the
PLAB class. If you think this is better, we can also use this for
pushing. I am tending to just removing it.

Thanks,
  Thomas