Integrated: 8318201: GenShen: Remove preparatory young GC before bootstrap

[Kelvin Nilsen]

We currently precede every old-mark bootstrap gc with a young gc.  The intent was to replenish the young allocation pool before beginning the "long" coalesce-and-fill effort which is part of the bootstrap gc.  This current design is mis-guided because the coalesce-and-fill effort will be preempted if a young gc is triggered while we are working on coalesce-and-fill.

This PR has the potential of reducing the number of concurrent young gcs by the number of times we initiate an old bootstrap gcs.

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Commit messages:
 - Remove preparatory young GC before boostrap cycle

Changes: https://git.openjdk.org/shenandoah/pull/340/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=340&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318201
  Stats: 23 lines in 1 file changed: 0 ins; 19 del; 4 mod
  Patch: https://git.openjdk.org/shenandoah/pull/340.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/340/head:pull/340

PR: https://git.openjdk.org/shenandoah/pull/340