Improve performance of changing mail message flag

Summary:
Skip the IMAP FETCHes when a message is marked/unmarked as seen or flagged.

This makes the most common scenarios of updating a mail message faster.

Reviewers: #syncroton_developers, mollekopf

Reviewed By: #syncroton_developers, mollekopf

Subscribers: #syncroton_developers

Differential Revision: https://git.kolab.org/D4406