RFR: Fix verification of remembered set at mark start [v2]

[Kelvin Nilsen]

> All objects residing between TAMS and top() within each old region are examined independent of the marking context.

Kelvin Nilsen has updated the pull request incrementally with two additional commits since the last revision:

 - Simplify rem-set verification code at init mark
   
   The code as originally written was mostly correct.  Use that
   implementation with just a few refinements to properly handle promotions
   that occur during concurrent old-gen marking.
 - Simplify the fix to rem-set verifier
   
   Just remove the offending assert().  The code as originally written
   should work ok.

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/197/files
  - new: https://git.openjdk.org/shenandoah/pull/197/files/e30a9aac..7a659985

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=197&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=197&range=00-01

  Stats: 47 lines in 1 file changed: 8 ins; 29 del; 10 mod
  Patch: https://git.openjdk.org/shenandoah/pull/197.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/197/head:pull/197

PR: https://git.openjdk.org/shenandoah/pull/197