[qtcontacts-sqlite] Detect partial-sync artifacts

This commit adds detection for artifacts of partially-successful
synchronisation cycles, to avoid creating local-side duplicates of
remotely-added contacts or server-side duplicates of locally-added
contacts due to sync failures late in the sync procedure (eg, where
upsyncs are paginated, for example, and connectivity is lost partway
through).
Merge pull request #146 from chriadam/noduplicates

[qtcontacts-sqlite] Detect partial-sync artifacts