RFR: Include mark worker performance in report for concurrent mark phase

[William Kemper]

This will allow us to detect an uneven distribution of work among the mark threads.

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Commit messages:
 - Include mark worker performance in report for concurrent mark phase

Changes: https://git.openjdk.java.net/shenandoah/pull/132/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=132&range=00
  Stats: 4 lines in 3 files changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/132.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/132/head:pull/132

PR: https://git.openjdk.java.net/shenandoah/pull/132