Support CONDSTORE IMAP extension

CONDSTORE extension allows incremental flags change synchronization. That means
we don't have to list all messages in every mailbox to check whether there were
any flags changes, we just ask for messages that have changed since last time.
This saves a lot of bandwidth and massively speeds up synchronization
(especially for large folders).

CONDSTORE is today supported by almost every modern IMAP server, but in case
it's not not supported, the task simply falls back to listing all messages and
comparing flags.

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