RFR: Reset task queue stats before rset scan

[William Kemper]

This change fixes an assert caused when task queue stats were reset after the rset scan has pushed oops into the queues (all of those "pushes" become uncounted).

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Commit messages:
 - Move concurrent rset scan to top level in phase timings report
 - Reset task queue stats before rset scan

Changes: https://git.openjdk.java.net/shenandoah/pull/69/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=69&range=00
  Stats: 12 lines in 3 files changed: 8 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/69.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/69/head:pull/69

PR: https://git.openjdk.java.net/shenandoah/pull/69