Explicitly set DB_ constants in loadOnly:true

[lucatume]

Fixes #753

This updates the WPLoader module, and its documentation, to make sure the database used when using the module in loadOnly:true mode will be the one specified by the dbUrl or other db configuration parameters.
The module will silence redeclarations of the DB_HOST, DB_USER, DB_PASSWORD and DB_NAME constants.
The module will also set the WPBROWSER_LOAD_ONLY env var to either 1 or 0 depending on the configuration of loadOnly to true or false respectively.

The latter should provide support for more advanced configurations and mitigations when WordPress set ups with stricter checking (e.g. Bedrock) would render the redefinition silencing useless.