RFR: Precleaning should use GC workers to do actual work

[Aleksey Shipilev]

After yesterday's String Dedup fixes, sh/jdk started to fail testing with:

#  Internal Error
(/home/jenkins/workspace/nightly/shenandoah-jdk/src/hotspot/share/gc/shenandoah/shenandoahThreadLocalData.hpp:111),
pid=136062, tid=136069
#  assert(thread->is_Worker_thread()) failed: Must be a worker thread
#

Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native
code)
V  [libjvm.so+0x1ad155d]  VMError::report_and_die(int, char const*, char const*, __va_list_tag*,
Thread*, unsigned char*, void*, void*, char const*, int, unsigned long)+0x15d
V  [libjvm.so+0x1ad24f7]  VMError::report_and_die(Thread*, void*, char const*, int, char const*,
char const*, __va_list_tag*)+0x47
V  [libjvm.so+0xbdfa79]  report_vm_error(char const*, int, char const*, char const*, ...)+0x109
V  [libjvm.so+0x184a346]  ShenandoahStringDedup::enqueue_candidate(oop)+0x806
V  [libjvm.so+0x17d2742]  void ShenandoahConcurrentMark::mark_through_ref<oop, (UpdateRefsMode)0,
(StringDedupMode)1>(oop*, ShenandoahHeap*, Padded<BufferedOverflowTaskQueue<ObjArrayChunkedTask,
(MemoryType)5, 131072u>, 128ul>*, ShenandoahMarkingContext*)+0x1a2
V  [libjvm.so+0x17d31bc]  ShenandoahSATBBufferClosure::do_buffer(void**, unsigned long)+0x11c
...
V  [libjvm.so+0x17c1e0a]  ShenandoahConcurrentMark::preclean_weak_refs()+0x22a
V  [libjvm.so+0x180ca50]  ShenandoahHeap::entry_preclean()+0xd0
V  [libjvm.so+0x17f9e28]  ShenandoahControlThread::service_concurrent_normal_cycle(GCCause::Cause)+0x1d8
V  [libjvm.so+0x17fb521]  ShenandoahControlThread::run_service()+0x861
V  [libjvm.so+0xb3f6e2]  ConcurrentGCThread::run()+0x92
V  [libjvm.so+0x15d4102]  thread_native_entry(Thread*)+0x112

It happens because pre-cleaning is performed by the ShControlThread itself, not GC worker. Which
does not have worker-id designation (or even GCLABs, for that matter). It was like that because
precleaning is not parallel in the ReferenceProcessor, but by now it is obvious to have drawbacks.
This patch moves precleaning to a single GC worker:
  http://cr.openjdk.java.net/~shade/shenandoah/preclean-worker/webrev.01/

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey