Postgres provisioning fixes

[andrewwhitehead]

For support of Postgres 15 where the CREATE permission isn't automatically granted on the 'public' schema.

Adds a new optional 'schema' parameter for postgres connection URLs (ie. postgres://user:pw@host/db?schema=public). When not set, the schema now defaults to the username when creating new tables.

For new stores,

• If the database is newly created, then the database owner is set explicitly
• A new schema is created in the database, with the same name as the user. This is consistent with the default search path: ('$user', public)
• All tables are created in the specified schema

For existing stores,

• If the schema isn't specified, then all schemas in the search path are checked for the config table (not just 'public')
• When the schema is set explicitly in the connection URL, the search path is restricted just to that schema

Fixes #129 ?

[andrewwhitehead]

#178 should fix the test failure on Windows and it might be best to merge that PR first.