Support CONDSTORE

Often, multiple IMAP (RFC 3501) clients need to coordinate changes to
a common IMAP mailbox. Examples include different clients working on
behalf of the same user, and multiple users accessing shared
mailboxes. These clients need a mechanism to synchronize state
changes for messages within the mailbox. They must be able to
guarantee that only one client can change message state (e.g.,
message flags) at any time. An example of such an application is use
of an IMAP mailbox as a message queue with multiple dequeueing
clients.

These days many clients work with the same mail server, your desktop, laptop, mobile devices are all retrieving mails and updating them. So, it would be safe to support this IMAP extension which can be used to avoid overwriting some other client's changes.

https://tools.ietf.org/html/rfc4551

Read RFC 4551 first and compare its requirements with the existing go-imap implementation. Done means the library supports the CONDSTORE extension described there, with the behavior verified by appropriate tests.

Written by the indexing model from the issue text.