[MTimeDB] Use exact match when exact match is requested

Summary:
As MDB_SET_RANGE moves the cursor to the first element greater or equal
the key, and updates the key in place, MTimeDB::get typically returned
the first newer document, and all with the same timestamp.

Test Plan: ctest

Reviewers: Baloo, Frameworks, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20330