Raise Dovecot client limits

roles/mail/templates/dovecot-frontend/conf.d/10-master.conf

@@ -119,9 +119,9 @@ service dict {
 
 # Bump anvil and ipc client limit to cater for many login processes
 service anvil {
-  client_limit = 4096
+  client_limit = 6247
 }
 
 service ipc {
-  client_limit = 4096
+  client_limit = 6244
 }