Reduce excessive gcstress mode logging.

Increase the default log thresholds by 10x in gcstress mode
to silence most logging (but still log pathological cases).

On the other hand, still log at least one GC per 10 seconds,
so that we preserve some informational value and progress.
Furthermore, the sampling is biased to reporting expensive GCs.

Bug: 170871390
Test: jdwp tests in gcstress mode.
Change-Id: I1747f0af3dcf313d9eb1a657bdfb8e0802f74993
4 files changed