RFR: Fix concurrent mark worker timings

[William Kemper]

When we added working timings for concurrent mark, they only reported the timing of the last iteration of the top level mark loop (which iteratively flushes SATB buffers on thread local handshakes). This changes adds an option to accumulate the workers' timing values.

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Commit messages:
 - Fix concurrent mark worker timings

Changes: https://git.openjdk.java.net/shenandoah/pull/133/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=133&range=00
  Stats: 6 lines in 3 files changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/133.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/133/head:pull/133

PR: https://git.openjdk.java.net/shenandoah/pull/133