Summary:
- Keys() functions in the indices can't be const because ConcurrentMap doesn't provide const accessors (and they are broken in skiplist) :D
- no cucumber tests because many tests create indices so it's hard to say what's inside and what not
Reviewers: buda, mislav.bradac
Reviewed By: mislav.bradac
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D797