Summary:
Permute query parts.
Permute matching only by selecting the starting node.
Flip the expansion when expanding from the other node.
Split planner into rule_based_planner and variable_start_planner
Use symbol hash when collecting expansion nodes
Multiple node atoms may point to the same symbol, and we could generate
multiple starting positions per atom which are the same. Using symbol
hash and equality prevents generating those redundant plans.
Correctly permute optional and merge matchings
Test VariableStartPlanner
Reviewers: florijan, mislav.bradac, buda, lion
Reviewed By: florijan, buda
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D417
