Guess which executor to use based on graph size

[j6k4m8]

This will have to get smarter once multiple graph databases are supported out of the box, but for now, small things can be run in memory and all larger things will need to use Neo4j.

[j6k4m8]

I think this is a wontfix. After some benchmarking tests, it looks like the interchange point where neo4j ingest becomes the driving time-consuming operation is dependent on both graph size as well as motif, which means there's no way to decide this a-priori (and certainly no way to decide this automatically based upon the haystack alone).