[crac] RFR: Wait until G1 GC has finished before creating a snapshot. [v7]

[Jan Kratochvil]

> @rvansa did report his snapshots are about 2x-3x bigger than they should be. He then also found it only happens if the snapshot is done too quickly after GC should have been run.
> 
> One can reproduce the race case by:
> 
> --- a/src/hotspot/share/gc/g1/g1UncommitRegionTask.hpp
> +++ b/src/hotspot/share/gc/g1/g1UncommitRegionTask.hpp
> @@ -35,7 +35,7 @@ class G1UncommitRegionTask : public G1ServiceTask {
>    // is short, while still making reasonable progress.
>    static const uint UncommitSizeLimit = 128 * M;
>    // Initial delay in milliseconds after GC before the regions are uncommitted.
> -  static const uint UncommitInitialDelayMs = 100;
> +  static const uint UncommitInitialDelayMs = 10*1000;
>    // The delay between two uncommit task executions.
>    static const uint UncommitTaskDelayMs = 10;

Jan Kratochvil has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Do not use synchronization, directly unmap it.
 - Merge branch 'crac' into crac-g1gcwait
 - Make -active volatile.
    - bugreported by Sergey Nazarkin
 - Remove accidental change of UncommitInitialDelayMs.
    - bugreported by Radim Vansa
 - Add an assertion at least for !is_ConcurrentGC_thread().
 - Remove calling thread state assertions for wait_for_collection_finish().
 - Add a generic virtual method wait_for_collection_finish().
    - based on a review by Radim Vansa.
   Assert calling thread state for wait_for_collection_finish().
    - based on a review by Radim Vansa.
 - Wait until G1 GC has finished before creating a snapshot.

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

Changes:
  - all: https://git.openjdk.org/crac/pull/93/files
  - new: https://git.openjdk.org/crac/pull/93/files/2eb4a7ee..355d8224

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=93&range=06
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=93&range=05-06

  Stats: 884 lines in 9 files changed: 416 ins; 444 del; 24 mod
  Patch: https://git.openjdk.org/crac/pull/93.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/93/head:pull/93

PR: https://git.openjdk.org/crac/pull/93