Populate service_password_hash at database setup time

(One paragraph: the idea.)

(Motivation, problem being solved, related context.)

The service_password_hash column (or equivalent credential store) for each service's database user is not populated by recreate_database.sh or setup_user.sql. Without it, service-to-database authentication via hashed passwords cannot work.

Update the setup scripts to:

1. Generate (or accept as a parameter) a password for each service DB user.
2. Hash it with the appropriate function (pgcrypto, bcrypt, or Postgres crypt).
3. Store the hash in the credential column at setup time.