IMAP data migrator and other improvements
Summary:
- IMAP data migration
- DAV can be used as an exporter (but this wasn't really tested yet)
- Async jobs can handle multiple items at once
- data:migrator --sync option
Reviewers: #hkccp_developers, mollekopf
Reviewed By: #hkccp_developers, mollekopf
Subscribers: mollekopf, #hkccp_developers
Differential Revision: https://git.kolab.org/D4845