Performance optimization when edge column and/or value are not needed

[li-boxuan]

IIRC JanusGraph always fetches the whole edge (column + value) even if you only need the count (in which case neither column nor value is really needed), or you only need edge id (in which case only column is needed).

There are two scenarios:

1. neither column nor value is needed. An example would be g.V(v).bothE().count(). Note that g.V(v).outE().count() does need "column" to retrieve direction info.
2. only column is needed. An example would be g.V(v).bothE().id(). See https://docs.janusgraph.org/advanced-topics/data-model/#individual-edge-layout on what info is stored in "column".

Inspired by https://groups.google.com/g/janusgraph-users/c/tFFI9ApCr-o/m/01gPsHECCQAJ