RFR: 8273000: Remove WeakReference-based class initialisation barrier implementation [v2]

[Vladimir Ivanov]

> Get rid of WeakReference-based logic in DirectMethodHandle::checkInitialized() and reimplement it with `Unsafe::ensureClassInitialized()`/`shouldBeInitialized()`. 
> 
> The key observation is that `Unsafe::ensureClassInitialized()` does not block the initializing thread. 
> 
> Also, removed `Unsafe::shouldBeInitialized()` in `DMH::shouldBeInitialized(MemberName)` to save on calling into the VM.
> `Unsafe::ensureClassInitialized()` already has a fast-path check which checks whether the class is fully initialized or not.
> 
> Testing: tier1 - tier6

Vladimir Ivanov has updated the pull request incrementally with one additional commit since the last revision:

  Improve comments

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5258/files
  - new: https://git.openjdk.java.net/jdk/pull/5258/files/9fc92167..64f2de83

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5258&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5258&range=00-01

  Stats: 13 lines in 2 files changed: 8 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5258.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5258/head:pull/5258

PR: https://git.openjdk.java.net/jdk/pull/5258