Bug #711053 - WebDAV: Do not overwrite UID of a contact

Apart of aforementioned fix to not overwrite contact UIDs, this also fixes:
- store href in a new vCard attribute, similar to etag
- possible slowness and busy loop of constant rewrite of a local cache on update
- removal of deleted contacts on the server from local cache
- preserve contact UID on create, if not used already
- be able to get all contacts with e_book_backend_cache_get_contacts()