Port partial commit e7bd563e228 for LUCENE-5644 for Lucene 4.8.1

Only including the PulseAll in this change; the rest will be in batch 5)

Author: paulirwin

src/Lucene.Net/Index/DocumentsWriterPerThreadPool.cs

@@ -283,6 +283,9 @@ internal void DeactivateUnreleasedStates()
                         threadState.Unlock();
                     }
                 }
+
+                // In case any threads are waiting for indexing:
+                UninterruptableMonitor.PulseAll(this);
             }
             finally
             {