RFR (M) JDK-8151178: Move the collection set out of the G1 collector policy

[Mikael Gerdin]

Hi all,

As part of an attempt to make it easier for G1 to contain multiple 
collector policies I suggest that the collection set code should be 
moved out of the current G1 collector policy class into a class of its own.

I've tried to hold back on doing further cleanups in this patch in order 
to make it easier to review the changes.

I do have some ideas on how the collection set code could be further 
cleaned up but I'm not sure if I will have time to test them properly 
before FC.

Webrev:
http://cr.openjdk.java.net/~mgerdin/8151178/version1/webrev_full

I've also split the change into three webrevs in the version1 directory:
webrev_1 consists of moving the code and changing code accessing 
collection set related methods
webrev_2 consists of renaming almost all methods in the new class to not 
contain redundant collection set naming
webrev_3 restores the functionality for collector policy extensions to 
hook into collection set selection.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8151178

Testing: JPRT, RBT GC testing.

Thanks6
/Mikael