RFR: 8341485: GenShen: Make evac tracker a non-product feature and confine it to generational mode

[William Kemper]

During the course of development of the generational mode, we added a feature to keep track of which threads were evacuating objects and how many evacuations lost the race. The instrumentation added unwarranted overhead. This change makes this instrumentation a non-product feature for the generational mode and removes it entirely from the non-generational modes.

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Commit messages:
 - Only instantiate evac tracker in generational mode
 - Only use evac tracker with generational mode
 - Do not retry with smaller lab, disable evac stats
 - Make evacuate_object non-virtual (this breaks genshen)

Changes: https://git.openjdk.org/shenandoah/pull/506/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=506&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341485
  Stats: 88 lines in 10 files changed: 33 ins; 28 del; 27 mod
  Patch: https://git.openjdk.org/shenandoah/pull/506.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/506/head:pull/506

PR: https://git.openjdk.org/shenandoah/pull/506