[Dovecot] METADATA for v2.2.7

[Timo Sirainen]

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)