Reduce contention on flushControl.isFullFlush(). (#12958)

`flushControl.isFullFlush()` is a surprising source of contention with
documents that are cheap to index and many indexing threads. If I slightly
modify luceneutil's `IndexGeoNames` benchmark to configure a 4GB indexing
buffer and disable `TextField` fields, which are more costly to index than
`KeywordField` or `IntField` fields, this brings the time to load all the
dataset in the `IndexWriter` buffers from 8.0s to 7.0s.

lucene/core/src/java/org/apache/lucene/index/DocumentsWriter.java

@@ -167,7 +167,11 @@ final class DocumentsWriter implements Closeable, Accountable {
   private boolean applyAllDeletes() throws IOException {
     final DocumentsWriterDeleteQueue deleteQueue = this.deleteQueue;
 
-    if (flushControl.isFullFlush() == false
+    // Check the applyAllDeletes flag first. This helps exit early most of the time without checking
+    // isFullFlush(), which takes a lock and introduces contention on small documents that are quick
+    // to index.
+    if (flushControl.getApplyAllDeletes()
+        && flushControl.isFullFlush() == false
         // never apply deletes during full flush this breaks happens before relationship.
         && deleteQueue.isOpen()
         // if it's closed then it's already fully applied and we have a new delete queue

lucene/core/src/java/org/apache/lucene/index/DocumentsWriterFlushControl.java

@@ -509,6 +509,14 @@ final class DocumentsWriterFlushControl implements Accountable, Closeable {
     return flushDeletes.getAndSet(false);
   }
 
+  /**
+   * Check whether deletes need to be applied. This can be used as a pre-flight check before calling
+   * {@link #getAndResetApplyAllDeletes()} to make sure that a single thread applies deletes.
+   */
+  public boolean getApplyAllDeletes() {
+    return flushDeletes.get();
+  }
+
   public void setApplyAllDeletes() {
     flushDeletes.set(true);
   }