RFR: 8287007: [cgroups] Consistently use stringStream throughout parsing code [v2]

[Severin Gehwolf]

> Please review this cleanup change in the cgroup subsystem which used to use hard-coded stack allocated
> buffers for concatenating strings in memory. We can use `stringStream` instead which doesn't have the issue
> of hard-coding maximum lengths (and related checks) and makes the code, thus, easier to read.
> 
> While at it, I've written basic `gtests` verifying current behaviour (and the same for the JDK code). From
> a functionality point of view this is a no-op (modulo the one bug in the substring case which seems to be
> there since day one). I'd prefer if we could defer any change in functionality to a separate bug as this is
> meant to only use stringStream throughout the cgroups code.
> 
> Testing:
> - [x] Container tests on Linux x86_64 cgroups v1 and cgroups v2
> - [x] Added tests, which I've verified also pass before the stringStream change
> 
> Thoughts?

Severin Gehwolf 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:

 - Merge branch 'master' into jdk-8287007-string-stream
 - Add cgroups v2 java test
 - use stringStream in cgroups v2
 - Add gtest for cgroups v2 code path
   
   Also fixes the bug when cgroup path is '/'.
 - 8287007: [cgroups] Consistently use stringStream throughout parsing code
 - 8287007: [cgroups] Consistently use stringStream throughout parsing code

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8969/files
  - new: https://git.openjdk.java.net/jdk/pull/8969/files/a7b156a4..8047fe37

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8969&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8969&range=00-01

  Stats: 60055 lines in 839 files changed: 34126 ins; 19620 del; 6309 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8969.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8969/head:pull/8969

PR: https://git.openjdk.java.net/jdk/pull/8969