this allows setting per-mailbox and per-server annotations (metadata). we have
a fixed maximum for total number of annotations (1000) and their total size
(1000000 bytes). this size isn't held against the regular quota for simplicity.
we send unsolicited metadata responses when a connection is in the idle
command and a change to a metadata item is made.
we currently only implement the /private/ namespace. we should implement the
/shared/ namespace, for mox-global metadata annotations. only the admin should
be able to configure those, probably through the config file, cli, or admin web
interface.
for issue #290
