Use max-mails setting with higher priority

The `mail-chunk-size` is set to its configured value or `max-mails`
whichever is lower.

modules/joex/src/main/resources/reference.conf

@@ -106,14 +106,18 @@ docspell.joex {
       max-folders = 50
 
       # How many mails (headers only) to retrieve in one chunk.
-      mail-chunk-size = 100
+      #
+      # If this is greater than `max-mails' it is set automatically to
+      # the value of `max-mails'.
+      mail-chunk-size = 50
 
       # A limit on how many mails to process in one job run. This is
-      # only to avoid resource allocation to one user/collective.
+      # meant to avoid too heavy resource allocation to one
+      # user/collective.
       #
       # If more than this number of mails is encountered, a warning is
       # logged.
-      max-mails = 1000
+      max-mails = 500
     }
   }