RFR (XS): 8145667: Move FromCardCache into separate files

[Thomas Schatzl]

Hi all,

  please review this change that moves the FromCardCache into separate
files. It is pretty much a "self-sustaining" part of remembered set
processing which I think is enough to warrant moving it into a separate
file.
Also it then does not stay in the way when working on remembered set code.

Follow-up CRs will rename it according to naming conventions
(->G1FromCardCache, JDK-8145671), and some clean-up to remove
dependencies to HeapRegionRemSet (JDK-8145672).

So this change is purely moving the class into separate files.

CR:
https://bugs.openjdk.java.net/browse/JDK-8145667
Webrev:
http://cr.openjdk.java.net/~tschatzl/8145667/webrev/
Testing:
compilation

Thanks,
  Thomas