[qtcontacts-sqlite] Minimize data extraction for sync process

Only load OOB data that is needed to test for changes until it becomes
necessary to retrieve the remainder for processing changes.
[qtcontacts-sqlite] Test for relevance to current account earlier

Avoid loading the stored OOB data until we have determined that there
exist changes that affect the current account.

Removes the possibility that we can inspect the previous version to
determine relevance, as this is not a feature currently used.
[qtcontacts-sqlite] Avoid unnecessary stores after sync

Do not update stored data that has not been modified.
[qtcontacts-sqlite] Simplify statements with temporary variables