RFR: 8224185: ZGC: Introduce "High Usage" rule

[Per Liden]

Start a GC cycle if the amount of free memory is 5% or less. This is a 
preventive measure in the case where the application has a very low 
allocation rate, such that the allocation rate rule doesn't trigger, but 
the amount of free memory is still slowly but surely heading towards 
zero. In this situation, we should start a GC cycle to avoid a potential 
allocation stall later.

Testing: New jtreg test added. Currently running tier 1-6.

Bug: https://bugs.openjdk.java.net/browse/JDK-8224185
Webrev: http://cr.openjdk.java.net/~pliden/8224185/webrev.0

/Per