Make the blocking/masking admin configs compatible with the swh db utilities

To be able to use the swh db command line, the configurations need to have the following shape:

<toplevel key>:
  cls: postgresql
  db: <postgresql DSN>

The usage of blocking_db and masking_db instead of plain db, and lack of overridability in the swh db utilities, made schema migrations more annoying than necessary.

The old configuration key still works but raises a Deprecation warning.

The configuration of the proxies themselves is unchanged, as having a differently-named key for the proxy-specific database makes sense to avoid confusion.