Fix failure to display subsequent pages in contacts

There is a bug in rcube_db::num_rows when used on a limitquery the query
has a record offset.

fix: removed all references to deprecated rcube_db::num_rows
replaced with db count where count is required (also less expensive).
removed counting where not required
Error when saving new contact

Fix: calling carddav_addressbook_sync  with null instead of false for
card_id prevents the sync to try an update on nonexisting card_id false.