RFR: 8364183: Shenandoah: Improve commit/uncommit handling

[Aleksey Shipilev]

While working on related bug, I found that Shenandoah commit/uncommit code has a few issues:
 1. It throws Java OOME on commit/uncommit failures. These should actually be VM-level out of memory errors, so they are properly fatal.
 2. Throwing OOME on uncommit looks fairly weird.
 3. We are not zapping on the newly committed regions. 

Additional testing:
 - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`
 - [ ] Linux x86_64 server fastdebug, `all`  + `-XX:+UseShenandoahGC`

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

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/26502/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26502&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364183
  Stats: 59 lines in 3 files changed: 20 ins; 14 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/26502.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26502/head:pull/26502

PR: https://git.openjdk.org/jdk/pull/26502