RFR: 8277807: Increase default initial concurrent refinement threshold

[Kim Barrett]

Please review this change to the default initial concurrent refinement
threshold.  The new value takes into account options that should provide a
starting point that is closer to the long-term range.

Testing:
mach5 tier1
manually verified logged initial value matched expectations.

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Commit messages:
 - increase default initial green zone value

Changes: https://git.openjdk.java.net/jdk/pull/6549/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6549&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277807
  Stats: 13 lines in 1 file changed: 8 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6549.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6549/head:pull/6549

PR: https://git.openjdk.java.net/jdk/pull/6549