RFR(S) 8154745: Move default G1 pause time target setup to argument parsing

[Mikael Gerdin]

Hi all,

Please review this fairly small change to move even more code out of 
G1CollectorPolicy.
The policy has previously initialized and validated MaxGCPauseMillis and 
GCPauseIntervalMillis in its constructor.

This should be moved to Arguments::set_g1_gc_flags, where other default 
values of globally known flags are set.
We can then move the constraint checking to the proper command line flag 
constraint functions (pause interval > pause target and setting pause 
interval without setting pause target) and modify the constraints so 
that they are applied before the GC subsystem is initialized (by 
changing AfterMemorInit to AfterErgo)

I also took the liberty to correct the comment I copied to correctly 
state that we set up the pause interval based on the pause time target, 
not the other way around.

Bug: https://bugs.openjdk.java.net/browse/JDK-8154745
Webrev: http://cr.openjdk.java.net/~mgerdin/8154745/webrev.0
Testing: RBT GC Testing

/Mikael