RFR: Clean up CondCardMark code: remove excess barriers, enable it by default [v2]

[Aleksey Shipilev]

> The `StoreLoad`, `StoreStore` barriers in that code are only needed for CMS, and does not relate to Shenandoah. Also, `UseCondCardMark` is a recommended option for most machines today, and therefore we better just enable it ergonomically.
> 
> Additional testing:
>  - [x] hotspot_gc_shenandoah, x86_64, {fastdebug,release}

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  More cleanups of scanned_concurrently

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

Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/26/files
  - new: https://git.openjdk.java.net/shenandoah/pull/26/files/6714df04..2668e01f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=26&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=26&range=00-01

  Stats: 15 lines in 3 files changed: 0 ins; 14 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/26.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/26/head:pull/26

PR: https://git.openjdk.java.net/shenandoah/pull/26