RFR: Shenandoah unified logging

[cfumiwu]

Writing the Shenandoah log file to the log stream without creating a new stream for it.

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Commit messages:
 - Delete the ShenandoahLogRegionSampling from the globals.hpp
 - Change the log target to debug tag
 - Using unified logging for shenandoah log file output

Changes: https://git.openjdk.org/shenandoah/pull/162/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=162&range=00
  Stats: 575 lines in 7 files changed: 27 ins; 542 del; 6 mod
  Patch: https://git.openjdk.org/shenandoah/pull/162.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/162/head:pull/162

PR: https://git.openjdk.org/shenandoah/pull/162