[MTimeDB] Allow timestamp newer than the newest document in range match

Summary:
In case the timestamp in a LessEqual match is newer than the newest
document, MDB_SET_RANGE returns MDB_NOTFOUND. Set the cursor to the last
entry.

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/D20331