RFR: 8078641: MethodHandle.asTypeCache can retain classes from unloading [v4]

[Vladimir Ivanov]

> `MethodHandle.asTypeCache` keeps a strong reference to adapted `MethodHandle` and it can introduce a class loader leak through its `MethodType`.
> 
> Proposed fix introduces a 2-level cache (1 element each) where 1st level can only contain `MethodHandle`s which are guaranteed to not introduce any dependencies on new class loaders compared to the original `MethodHandle`. 2nd level is backed by a `SoftReference` and is used as a backup when the result of `MethodHandle.asType()` conversion can't populate the higher level cache.  
> 
> The fix is based on [the work](http://cr.openjdk.java.net/~plevart/jdk9-dev/MethodHandle.asTypeCacheLeak/) made by Peter Levart @plevart back in 2015.
> 
> Testing: tier1 - tier6

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

  Address review comments

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5246/files
  - new: https://git.openjdk.java.net/jdk/pull/5246/files/7a87aee3..365dd454

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5246&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5246&range=02-03

  Stats: 5 lines in 1 file changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5246.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5246/head:pull/5246

PR: https://git.openjdk.java.net/jdk/pull/5246