Slightly faster UDS constructor

Summary:
Don't use the deprecated uds insert() method,
call directly to the UDSEntryPrivate reserve() and insert() methods.

Test Plan: auto tests pass.

Reviewers: dfaure, Frameworks

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14231