* chore: don't support persistence for memory backend
* chore: setup graph by first step graph when graph is null
* chore: apply relations optimize logic
* chore: don't support null properties
* fix: g.V().has(empty).count() and g.V().count().as('a').select('a')
* fix: specify the code generation directory for raft.proto (#2057)
* fix: tinkerpop unit schema no define
* fix the style & clean some outdated code

Co-authored-by: Simon Cheung <ming@apache.org>
Co-authored-by: imbajin <jin@apache.org>