Move can-match thread from transport thread to search coordinator thread

Author: chrisparrinello

server/src/main/java/org/elasticsearch/action/search/SearchTransportService.java

@@ -439,7 +439,7 @@ public static void registerRequestHandler(TransportService transportService, Sea
 
         transportService.registerRequestHandler(
             QUERY_ACTION_NAME,
-            EsExecutors.DIRECT_EXECUTOR_SERVICE,
+            transportService.getThreadPool().executor(ThreadPool.Names.SEARCH_COORDINATION),
             ShardSearchRequest::new,
             (request, channel, task) -> searchService.executeQueryPhase(
                 request,