RFR: 8297718: Make NMT free:ing protocol more granular

[Johan Sjölen]

This PR avoids the double registration of a failed `os::realloc` in NMT by doing the NMT free protocol steps inline (asserting block integrity, marking of MallocHeader, and registrering results to statistics). This is done by creating a new struct `FreePackage` which includes all of the necessary data for registrering results.

I also did some light refactoring which I think makes the protocol clearer, these are done piece wise as separate commits for the reviewers' convenience.

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Commit messages:
 - Add MemTracker::record_free()
 - Rename record_free_header() to record_free()
 - Rename record_free to record_free_block
 - Rename MallocHeader::free_recording_data() to MallocHeader::free_package()
 - Assert NMT is enabled in MemTracker::record_free()
 - Open up protocol

Changes: https://git.openjdk.org/jdk/pull/11390/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11390&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297718
  Stats: 62 lines in 6 files changed: 34 ins; 7 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/11390.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11390/head:pull/11390

PR: https://git.openjdk.org/jdk/pull/11390