Make sure there is only one MTime per document in the MTimeDB

Summary:
Previously, every time a file was was modified in place (e.g. appended),
the document was added for the new mtime, but never removed for the
old one.

Depends on: D16999

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D17000