ENH: refactor md schema

[chiahaoliu]

we need to revisit md schema based on current software development progress. Modification needed are:

1. remove dark_client/server_uid
2. change md_schema_md_version to xpdacq_md_version
3. include analysis_stage to specify at which level documents are being processed.
4. adjust required fields for validation if needed.

[CJ-Wright]

👍