RFR: 8264311: Heap object statistics

[Roman Kennke]

For Lilliput evaluation, it would be useful to be able to get some statistics about heap objects, e.g. how many objects there typically are, what is their (average) size, how big is the live data set, how many objecs have an identity hash code and how many objects are locked. Some of that information may be useful for general purpose too, e.g. the avg live data set and object size/count information might be quite useful to have.
Testing:
 - [x] Some manual tests, verifying the plausability of the output by hand
 - [ ] tier1
 - [ ] tier2

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Commit messages:
 - 8264311: Heap object statistics

Changes: https://git.openjdk.java.net/jdk/pull/3271/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3271&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264311
  Stats: 255 lines in 5 files changed: 255 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3271.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3271/head:pull/3271

PR: https://git.openjdk.java.net/jdk/pull/3271