RFR: 8259231: Fix the chance to allocate failure and improve the speed and quality of EpsilonHeap::allocate_work [v3]

[Lehua Ding]

> Hi all,
> 
> The `EpsilonHeap::allocate_work` method maybe can be fixed and improved by this:
> 1. it can prevent allocate failure by retry `_space->par_allocate` before expanding virtual space,  when there not enough virtual space but another thread expanding succeeded just and has enough space.
> 2. it can reduce the lock time by move `res = _space->par_allocate(size);` out of lock scope.
> 
> Test on macosx-x86_64-server-{release, fastdebug, slowdebug} with current test case.

Lehua Ding has updated the pull request incrementally with one additional commit since the last revision:

  Epsilon: clean trailing whitespace

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1794/files
  - new: https://git.openjdk.java.net/jdk/pull/1794/files/188c41a1..4ae547f0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1794&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1794&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1794.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1794/head:pull/1794

PR: https://git.openjdk.java.net/jdk/pull/1794