RFR (M): 8136681: Factor out IHOP calculation from G1CollectorPolicy

[Thomas Schatzl]

Hi all,

  can I have reviews for this preparatory change for adaptive IHOP
sizing, that factors out the IHOP calculation from G1Collectorpolicy
into separate classes?

So instead of having this single line at G1CollectorPolicy:944

 944   size_t marking_initiating_used_threshold =
 945     (_g1->capacity() / 100) * InitiatingHeapOccupancyPercent;
[...]
 949   if ((cur_used_bytes + alloc_byte_size) >
marking_initiating_used_threshold) {

that determines whether marking should start, that decision is
externalized into a G1IHOPControl class with a
get_conc_mark_start_threshold() method.

This allows flexible replacement of the IHOP calculation algorithm
depending on some switch using a factory method. 

In addition to that, this change tracks and passes several information
IHOPControl instances need, like allocation between gcs, mutator time
from initial mark to the first mixed gc, and others.

The change also adds some test.

It depends on all my recent patches in this area, namely:
8140689: Skip last young-only gc if nothing to do in the mixed gc phase
8140597: Forcing an initial mark causes G1 to abort mixed collections
8140585: PLAB statistics are flushed too late
8139874: After G1 Full GC, the next GC is always a young-only GC
8138740: Start initial mark right after mixed GC if needed

The problem is that it expects (and verifies) that the GC pauses are
executed in the "correct" order.

There is no behavioral change in this CR.

CR:
https://bugs.openjdk.java.net/browse/JDK-8136681
Webrev:
http://cr.openjdk.java.net/~tschatzl/8136681/webrev/
Testing:
jprt, vm.gc, lots of manual testing, new VM test

Note that SQE will create additional tests.

Thanks,
  Thomas