RFR: JDK-8256725: Metaspace: better blocktree and binlist asserts [v4]

[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!

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  Review feedback Richard+Leo

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1339/files
  - new: https://git.openjdk.java.net/jdk/pull/1339/files/4e85fe65..386550bb

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1339&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1339&range=02-03

  Stats: 49 lines in 3 files changed: 27 ins; 6 del; 16 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