[qtcontacts-sqlite] Add function to fetch OOB keys

Fetching both keys and values can be wastefully expensive, if only
the key values are needed.
[qtcontacts-sqlite] Automatic compression for OOB data

On devices, I/O costs are more problematic than CPU usage. Therefore,
it is preferable to compress OOB data where possible, to reduce the
amount of I/O it involves, at the cost of compression effort.
Merge pull request #151 from matthewvogt/compress-oob

[qtcontacts-sqlite] Automatic compression for OOB data