RFR: 8252049: Native memory leak in ciMethodData ctor [v2]

[Vladimir Ivanov]

> `ciMethodData` embeds `MethodData` to snapshot a state from the original `MethodData` instance.
> But `MethodData` embeds a `Mutex` which allocates a platform-specific implementation on C-heap.
> The `Mutex` is overwritten with `0`s, but the resources aren't deallocated, so the leak occurs.
> 
> Proposed fix is to run Mutex destructor right away. 
> 
> Initially, I thought about switching to `Mutex*`, but then I found that Coleen already tried that and observed a performance regression [1]. So, for now I chose the conservative approach.
> 
> In the longer term, I would consider replacing `MethodData::_extra_data_lock` with a lock-free scheme. Having a lock-per-MDO looks kind of excessive.
> 
> Testing:
> - [x] verified that no memory leak observed with the reported test
> - [x] tier1-4
> 
> [1] https://mail.openjdk.java.net/pipermail/hotspot-dev/2019-October/039783.html

Vladimir Ivanov has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Introduce MethodData::MDHeader class
 - Merge branch 'master' into 8252049.md.leak
 - Cleanup
 - Merge branch 'master' into 8252049.md.leak
 - Minor refactoring
 - Fix native memory leak in ciMethodData ctor

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1478/files
  - new: https://git.openjdk.java.net/jdk/pull/1478/files/8d429ef9..011b26ca

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1478&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1478&range=00-01

  Stats: 32207 lines in 828 files changed: 19824 ins; 4170 del; 8213 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1478.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1478/head:pull/1478

PR: https://git.openjdk.java.net/jdk/pull/1478