[Dovecot] METADATA for v2.2.7

In case someone wants to try: http://hg.dovecot.org/dovecot-2.2/rev/0a08efeb3f40

The commit message lists the TODO. So for example if you want to enable it just for webmail (because quota limits aren’t there yet):

mail_attribute_dict = file:%h/dovecot-attributes local 10.0.0.0/24 { imap_metadata = yes }

Compared to the old metadata plugin, there are some improvements:

• Uses lib-storage attributes internally, which means:
  • dsync replicates METADATA
  • it will be easy to add support for untagged METADATA notifications to clients when they change
• SETMETADATA atomically either updates all or none of the changes

Things that work in the METADATA code, but still require lib-dict changes to work properly:

• Values with unlimited sizes
• Values with binary data (NULs)