lib-imap-storage: Created new METADATA API.

lib-imap-storage: imap-msgpart rewrite and API change. The new API allows first parsing the validity of section strings and later relying on them being valid without having to re-parse it. The implementation also fixes a few things and adds "partial fetch cache".

Created lib-imap-storage for IMAP-specific functionality that uses lib-storage. This is done to prevent introducing dependencies on lib-storage in lib-imap. It contains IMAP message part access functionality, including URL-based access.