Implement "iterate" command in metadata server

Otherwise Dovecot times out when trying to iterate over metadata
of the folder. Apparently it happens when attempting to delete
folder from the server over IMAP.

cmdeploy/src/cmdeploy/dovecot/dovecot.conf.j2

@@ -13,12 +13,6 @@ auth_cache_size = 100M
 mail_debug = yes
 {% endif %}
 
-# Prevent warnings similar to:
-#   config: Warning: service auth { client_limit=1000 } is lower than required under max. load (10200). Counted for protocol services with service_count != 1: service lmtp { process_limit=100 } + service imap-urlauth-login { process_limit=100 } + service imap-login { process_limit=10000 }
-#   config: Warning: service anvil { client_limit=1000 } is lower than required under max. load (10103). Counted with: service imap-urlauth-login { process_limit=100 } + service imap-login { process_limit=10000 } + service auth { process_limit=1 }
-#   master: Warning: service(stats): client_limit (1000) reached, client connections are being dropped
-default_client_limit = 20000
-
 mail_server_admin = mailto:root@{{ config.mail_domain }}
 mail_server_comment = Chatmail server