Use Blob layer to save SchemaManager metadata

As a relatively quick fix for the individual value size restriction with respect to metadata.

The recent BlobAsync port of the Python layer was copied as-is. We'll most likely need to make changes if/when we use it for generic blob support but it seems to work fine for this use case.

Notably, LotsOfTablesDT, which creates 2000 tables in a single schema, passes again.