mu4e: support mu indexing performance options

Add two variables mu4e-index-cleanup and mu4e-index-lazy-check, which
correspond to mu index option --lazy-check and --nocleanup.

Extend the mu server protocol a bit to handle this.

The defaults keep things behaving as they done before.

mu4e/mu4e-vars.el

@@ -90,6 +90,27 @@ background."
   :group 'mu4e
   :safe 'booleanp)
 
+(defcustom mu4e-index-cleanup t
+  "Whether to run a cleanup face after indexing -- that is, see
+if the is a message in the filesystem for each file in the
+message store. Having this option as `t' ensures that no
+non-existing mesages are shown but can also be quite slow with
+large message stores."
+  :type 'boolean
+  :group 'mu4e
+  :safe 'booleanp)
+
+(defcustom mu4e-index-lazy-check nil
+  "Whether to run do a 'lazy check' for deciding whether to
+indexing a message. When this is set to `t', mu only uses the
+directory timestamps to decide on whether it needs to check the
+messages beneath it, which would miss messages that are modified
+outside mu. On the other hand, it's significantly faster."
+  :type 'boolean
+  :group 'mu4e
+  :safe 'booleanp)
+
+
 (defcustom mu4e-update-interval nil
   "Number of seconds between automatic calls to retrieve mail and
 update the database. If nil, don't update automatically. Note,