RFR: 8209346: Refactor SATBMarkQueue filter configuration

[Kim Barrett]

Please review this change to how SATBMarkQueue determines whether a
post-filtered buffer should be enqueued or continue to be used.

The qset's initialize function is now called with an enqueue threshold
percentage; G1 uses G1SATBBufferEnqueuingThresholdPercentage.  This
value is translated by initialize into a threshold index recorded in
the qset.

The queue's should_enqueue_buffer now obtains that threshold index and
compares it to the post-filter index to determine whether the buffer
should be enqueued or continue to be used.

This changeset is based on the in-review changes for JDK-8209396 and
JDK-8209345.
http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2018-August/022868.html
http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2018-August/022869.html

CR:
https://bugs.openjdk.java.net/browse/JDK-8209346

Webrev:
http://cr.openjdk.java.net/~kbarrett/8209346/open.00/

Testing:
Mach5 tier1-3, hs-tier4-5, in conjunction with JDK-8209345 and JDK-8209347.
Locally tier1 and hotspot_gc without those other changes.