[Transaction] Replace template for functor with std::function

Summary:
The callback parameter has a specific type, it accepts a document ID and
returns bool, while the template is overly broad.

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21703