RFR: JDK-8256725: Metaspace: better blocktree and binlist asserts

[Thomas Stuefe]

Hi,

may I have reviews please for this small change. 

To analyze JDK-8256572, I'd like to see more information in asserts for binlist and blocktree.

This patch:

- beefs up assertion messages when verifying binlist and blocktree
- adds a canary to the blocktree node to detect overwriters
- improves the blocktree printing
- adds a gtest death test to test the overwrite detection.

Thanks!

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Commit messages:
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/1339/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1339&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256725
  Stats: 120 lines in 5 files changed: 92 ins; 9 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1339.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1339/head:pull/1339

PR: https://git.openjdk.java.net/jdk/pull/1339