Integrated: Mutator may fail to evacuate

[Kelvin Nilsen]

In the case that a mutator thread fails to evacuate, this patch transitions the Java thread into a VM thread before blocking the thread.  Otherwise, an assertion error is raised.

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Commit messages:
 - Remove gratuitous new line
 - Undo JRT_ENTRY prologue attempts
 - Retreat to original solution
 - Let write_ref_field_pre_entry use JRT_LEAF protocol
 - Change LRB implementations for mutators
 - is_Java_thread() does not imply thread->thread_state() == thread_in_Java
 - Transition Java thread to VM thread Before Blocking Thread

Changes: https://git.openjdk.java.net/shenandoah/pull/117/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=117&range=00
  Stats: 12 lines in 1 file changed: 8 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/117.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/117/head:pull/117

PR: https://git.openjdk.java.net/shenandoah/pull/117