[buteo-sync-plugins] Retrieve contacts from non-privileged database.

This ensures we only export contacts that should be exportable to
external devices. The "local" syncTarget filter is no longer necessary
as using this database ensures we only export contacts that we should
export anyway.
Merge pull request #28 from nemomobile/use-nonpriv-db

[buteo-sync-plugins] Retrieve contacts from non-privileged database.