[carddav] Use Buteo logging macros for debug / warning output

This commit replaces usages of qWarning and qDebug with the logging
macros defined in the Buteo library, to ensure that logging output
is runtime-selectable.
[carddav] Detect server-side additions as previously upsynced

This commit ensures that if a server-side addition is reported, we
check to see if it was actually a previously-upsynced local contact,
so that we can set the correct contact id in it, for delta detection.