Move DataFusion SessionContext into DeltaLake and register tables automatically

[CGodiksen]

Currently we have a SessionContext in Context, in TableMetadataManager, and in MetadataManager. Each of these register tables manually on start up and when tables are created. Instead we should move SessionContext into DeltaLake so registering and de-registering tables is done automatically.

This implementation can also include other smaller features that were considered when implementing the current version of DeltaLake.

• Use separate schemas in SessionContext for tables and metadata tables to avoid naming conflicts.
• Put DeltaLake in an Arc or Rc and share it between data and metadata.
• Check if "duplicated" functionality can be removed from DeltaLake by automatically checking if a table_name is for a normal/model table or for a metadata table.
• Look into if a single TableProvider can be used for metadata tables and normal tables.
• Add unit testing to DeltaLake.

[skejserjensen]

Closed as most of it is implemented by #355 and the remaining bullets are superseded by #302 and #357.