RFR: 8287007: [cgroups] Consistently use stringStream throughout parsing code [v3]

[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 incrementally with one additional commit since the last revision:

  Remove fix for substring match

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8969/files
  - new: https://git.openjdk.java.net/jdk/pull/8969/files/8047fe37..84d952b8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8969&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8969&range=01-02

  Stats: 10 lines in 2 files changed: 0 ins; 7 del; 3 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