## Purpose of the PR
Subtask of #2265.
Adjust the project structure of this repository to include three sub-modules: hugegraph-server, hugegraph-pd, hugegraph-store at the root level.
## Main Changes
Roll back to the moment when https://github.com/apache/incubator-hugegraph/pull/2266 was merged on `pd-store` and incorporate the latest changes in `master`.
For more detailed information, please refer to https://github.com/apache/incubator-hugegraph/pull/2266#issue-1834369489.
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Co-authored-by: M <87920097+msgui@users.noreply.github.com>
* 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* fix the style & clean some outdated code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>
Peng Junzhi