RFR: Lower logging of collection set details to debug and use cset tag

[William Kemper]

This logging was unintentionally left at Info level.

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Commit messages:
 - Set cset info to Debug and use cset tag

Changes: https://git.openjdk.org/shenandoah/pull/232/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=232&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/232.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/232/head:pull/232

PR: https://git.openjdk.org/shenandoah/pull/232