RFR: 8275185: Remove dead code and clean up jvmstat LocalVmManager

[Ioi Lam]

LocalVmManager and PerfDataFile have APIs that are supposed to look for VMs owned by a specific user. No one uses these APIs, and they don't work anyway.

The current code is very confusing to look at. Since we're likely to change code in this area for further container support, it's better to clean up the code now.

- Remove all APIs that take a user name
- Also removed PerfDataFile.getFile() methods that are unused
- Cleaned up the code that looks up the hsperfdata_xxx files
  - Fix comments to explain what's happening
  - Avoid using Matcher.reset which is not thread-safe
  - Renamed confusing variables such as `userFilter` to make the code more readable
  - LocalVmManager.activeVms() probably doesn't need to be synchronized, but I kept it anyway to avoid unnecessary risks.

Testing with Oracle CI: tiers1-4, plus hs-tier5-rt (which tests containers and have extensive use of the management tools).

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Commit messages:
 - 8275185: Remove dead code and clean up jvmstat LocalVmManager

Changes: https://git.openjdk.java.net/jdk/pull/5923/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5923&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275185
  Stats: 319 lines in 2 files changed: 9 ins; 278 del; 32 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5923.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5923/head:pull/5923

PR: https://git.openjdk.java.net/jdk/pull/5923