Summary:
This diff consolidates local and remote update handling. It ensures and
tests that updates for remote elements are visible locally (on the
updating worker).
The next part will be accumulating remote updates and applying them on
the owner.
Also extracted a common testing fixture.
Reviewers: dgleich, buda, mtomic
Reviewed By: mtomic
Subscribers: mtomic, pullbot
Differential Revision: https://phabricator.memgraph.io/D1169
