RFR: Comprehensible GC trigger logging

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/compreh-trigger/webrev.01/

Our current logs are confusing if you want to figure out why exactly the GC had triggered. We need
to make them more comprehensible and user-friendly. It does so by telling exactly why the control
code decided to trigger the GC. It also does it with "gc" tag, not with "gc+ergo", because that bit
is the most important bit that heuristics tell.

The patch also eliminates ShenandoahAllocThreshold handling from free-threshold-based heuristics,
for simplicity. The value for that option is 0 by default, so it has no effect anyway.

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey