Add withEngineOrNullIfBeingReset

And deprecate old style getEngine/OrNull methods.

Apply new functionality to several methods that do not need to
wait for the engine being reset and can do with a null engine.
These pertain typically to periodic operations that can skip
a shard being reset and revisit it next time.

Also return empty stats for a few stats in case the engine is
being reset. These are stats that are already returned empty
from a hollow engine.

Relates ES-11457

Author: kingherc

server/src/main/java/org/elasticsearch/action/admin/indices/flush/FlushRequest.java

@@ -45,16 +45,16 @@ public FlushRequest(StreamInput in) throws IOException {
     }
 
     /**
-     * Returns {@code true} iff a flush should block
-     * if a another flush operation is already running. Otherwise {@code false}
+     * Returns {@code true} iff a flush should block (e.g., if the engine is being reset or
+     * if a another flush operation is already running). Otherwise {@code false}
      */
     public boolean waitIfOngoing() {
         return this.waitIfOngoing;
     }
 
     /**
-     * if set to {@code true} the flush will block
-     * if a another flush operation is already running until the flush can be performed.
+     * if set to {@code true} the flush will block (e.g., if the engine is being reset or
+     * if a another flush operation is already running) until the flush can be performed.
      * The default is <code>true</code>
      */
     public FlushRequest waitIfOngoing(boolean waitIfOngoing) {