Check how CONDSTORE/QRESYNC can be supported

[r10s]

This issue is here to collect ideas and information regarding the CONDSTORE/QRESYNC IMAP-extensions (see RFC 7162).

In general, both extensions seems to be well spreaded, eg. Dovecot has both enabled by default. However, as there may be servers that do not support them or the user may have disabled them, this can only be an optimization (eg. gmail shows CONDSTORE but not QRESYNC in its CAPABILITY string).

A high-level overview:

• conditional Store ("CONDSTORE") facility allows a client to quickly resynchronize mailbox flag changes. In case of Delta, these are mainly the $MDNSent and \Deleted flags that may be modified by another Delta Chat/another MUA and should be synchronized, see Improve multi-device support #52

• QRESYNC is an extension to CONDSTORE that allows a reconnecting client to perform resynchronization to CONDSTORE with less network activity.

Open Questions:

• How is this related to the IDLE-command? Will IDLE be interrupted when the flags for a message change?
• in my understanding QRESYNC only helps with CONDSTORE and does not save commands that are currently used for handling IMAP; QRESYNC does not allow a general approach to send multiple commands at once. but i am not sure about this.

[r10s]

closing this for now, no actionable item currently.