RFR: 8259231: Fix the chance to allocate failure and improve the speed and quality of EpsilonHeap::allocate_work [v2]

[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 two additional commits since the last revision:

 - Epsilon: revert irrelevant typo fixes
 - Epsilon: refactor to a cleaner form

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1794/files
  - new: https://git.openjdk.java.net/jdk/pull/1794/files/7c97808d..188c41a1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1794&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1794&range=00-01

  Stats: 14 lines in 2 files changed: 6 ins; 3 del; 5 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