Summary:
It occurred that part of the durability flakyness test might be that the
same durability directory is used always. If the test is run
simultaneously on a single system, there will be interference.
This might not actually fix all the flakyness :(
I also made the `utils::RandomString` function since that's now used in
multiple places, tested it etc.
Reviewers: buda, dgleich
Reviewed By: dgleich
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D1020
