better handle maildir cache

- get an updated maildir list after indexing
- add mu4e-added items to the list opportunistically

Remove mu4e-clear-caches / mu4e-cache-maildir-list to mu4e-obsolete.el

Fixes #2537.

mu4e/mu4e-server.el

@@ -643,7 +643,13 @@ the directory time stamp."
 
 (defun mu4e--server-mkdir (path &optional update)
   "Create a new maildir-directory at filesystem PATH.
-When UPDATE is non-nil, send a update when completed."
+When UPDATE is non-nil, send a update when completed.
+PATH must be below the root-maildir."
+  ;; handle maildir cache
+  (if (not (string-prefix-p (mu4e-root-maildir) path))
+      (mu4e-error "Cannot create maildir outside root-maildir")
+    (add-to-list 'mu4e-maildir-list ;; update cache
+                 (substring path (length (mu4e-root-maildir)))))
   (mu4e--server-call-mu `(mkdir
                           :path ,path
                           :update ,(or update nil))))