Stop using database connection strings

[moseshll]

Instead the user, password, host, db name, and any other parameters should be passed in to the Connection initializer as keyword args, possibly augmented with ENV and defaults, and passed as keyword arguments to Sequel.

Using a connection string/URI as the main parameter shifts the responsibility for percent-escaping credentials onto the caller.

See https://github.com/hathitrust/rights_database/blob/main/lib/rights_database/db.rb for a representative implementation.

The connection string may be maintained for backward compatibility but should be deprecated and should loudly say so if possible.

[aelkiss]

Completed by 4d6218d / #22