RFR: 8334491: GenShen: Revert changes to Shenandoah defaults

[William Kemper]

On the genshen branch, we have changed the adaptive heuristics decay factor to 0.1 and increased the learning cycles to 10. Testing with the original defaults of 0.5 and 5 (respectively) shows modest performance improvements in specjbb and extremem.

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Commit messages:
 - Sync decay factor, learning steps with upstream

Changes: https://git.openjdk.org/shenandoah/pull/450/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=450&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334491
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/450.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/450/head:pull/450

PR: https://git.openjdk.org/shenandoah/pull/450