[master] RFR: Use hashtable for obj->edge mapping in JFR, instead of mark-word

[Roman Kennke]

JFR overrides the mark-word to (temporarily) store a mapping from object to Edge*. This disturbs the Klass* that we need while tracing for leaks and for later emitting object information. Let's use a hashtable for this, instead, and leave the upper half of the mark-word alone. (The lower half is still used for marking.)

Testing:
 - [x] tier1
 - [x] tier2
 - [x] jdk/jfr (together with #17 most tests pass, needs one more follow-up test fix)

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Commit messages:
 - Use hashtable for obj->edge mapping in JFR, instead of mark-word

Changes: https://git.openjdk.java.net/lilliput/pull/18/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lilliput&pr=18&range=00
  Stats: 61 lines in 4 files changed: 54 ins; 2 del; 5 mod
  Patch: https://git.openjdk.java.net/lilliput/pull/18.diff
  Fetch: git fetch https://git.openjdk.java.net/lilliput pull/18/head:pull/18

PR: https://git.openjdk.java.net/lilliput/pull/18