[qtcontacts-sqlite] Properly enforce uniqueness constraints

Uniqueness constraints on details were previously not enforced
correctly.  This commit adds uniqueness constraint checks, and adds
a unit test to ensure we don't regress.

Also improve the aggregation unit tests to ensure that only one local
contact is ever created for new aggregate contact details.
Merge pull request #5 from chriadam/master

[qtcontacts-sqlite] Improve aggregation unit tests