RFR: Mixed evacuation [v3]

[Kelvin Nilsen]

> This code adds to generational Shenandoah the ability to perform concurrent garbage collection of young-gen and old-gen memory. Following completion of an old-gen concurrent marking effort, we select certain old-gen heap regions to serve as candidates for future collection sets. All dead objects within the old-gen heap regions that are not part of this candidate set are coalesced and filled so that remembered-set scanning of these old-gen heap regions will not be confused by "zombie objects" (objects that old-gen has decided are dead, which reside in regions that have not yet been collected). After concurrently coalescing and filling these dead objects, each subsequent young-gen evacuation pass includes a subset of the old-gen candidates until all candidates have been collected. This code passes TIER1 and hotspot-gc-shenandoah jtreg tests without regressions. A new jtreg test has been added to exercise concurrent old/young GC.

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

 - Add collector statistics tracking
   
   As requested by reviewer feedback for draft upstream pull request,
   ShenandoahOldGC::entry_coalesce_and_fill() now gathers additional
   collector statistics.
 - Improve coalesce and fill
   
   In old-gen memory ranges of HeapRegions not selected for collection,
   use marking_context->get_next_marked_addr() instead of visiting each unmarked
   object independently when deciding which ranges of memory to coalesce and
   fill.  This change builds and runs jtreg test=TIER1 and
   test=hotspot_gc_shenandoah wihtout regressions.
 - Fix trailing whitespace error and correct spelling errors in comment

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/29/files
  - new: https://git.openjdk.java.net/shenandoah/pull/29/files/50ffb169..76213506

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=29&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=29&range=01-02

  Stats: 71 lines in 3 files changed: 4 ins; 40 del; 27 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/29.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/29/head:pull/29

PR: https://git.openjdk.java.net/shenandoah/pull/29