Revert "Don't eagerly store articles in the Agent by default"

This reverts commit def34a2076.

This made storing articles in the Agent very slow.

lisp/gnus/gnus-agent.el

@@ -3812,7 +3812,6 @@ has been fetched."
         t))))
 
 (defun gnus-agent-store-article (article group)
-  (declare (obsolete nil "28.1"))
   (let* ((gnus-command-method (gnus-find-method-for-group group))
 	 (file (gnus-agent-article-name (number-to-string article) group))
 	 (file-name-coding-system nnmail-pathname-coding-system)

lisp/gnus/gnus-art.el

@@ -7092,7 +7092,10 @@ If given a prefix, show the hidden text instead."
 					      gnus-summary-buffer)
 		    (when gnus-keep-backlog
 		      (gnus-backlog-enter-article
-		       group article (current-buffer))))
+		       group article (current-buffer)))
+		    (when (and gnus-agent
+			       (gnus-agent-group-covered-p group))
+		      (gnus-agent-store-article article group)))
 		  (setq result 'article))
 		 (methods
 		  (setq gnus-override-method (pop methods)))

lisp/gnus/gnus-async.el

@@ -225,6 +225,12 @@ that was fetched."
       (save-excursion
 	(save-restriction
 	  (narrow-to-region mark (point-max))
+	  ;; Put the articles into the agent, if they aren't already.
+	  (when (and gnus-agent
+		     (gnus-agent-group-covered-p group))
+	    (save-restriction
+	      (narrow-to-region mark (point-max))
+	      (gnus-agent-store-article article group)))
 	  ;; Prefetch images for the groups that want that.
 	  (when (fboundp 'gnus-html-prefetch-images)
 	    (gnus-html-prefetch-images summary))