Use own database tables and follow Drupal coding standards

Problem/Motivation
- The module shares the fs_authmap table with other modules and uses the fs_salesforce_auth table. Uninstalling one module can break the other, and the names don't follow the Drupal convention of starting table names with the module name.
- generate_login_link() doesn't start with the module name and can clash with functions from other modules.
- The SFID class name and several class comments don't follow Drupal coding standards, and RestClient calls \Drupal::time() instead of using dependency injection.
- README.txt still describes Drupal 7.

Proposed resolution
- Move data to salesforce_auth_user and salesforce_auth_authmap with an update hook.
- Rename generate_login_link() to salesforce_auth_generate_login_link() and SFID to SalesforceId.
- Inject the time service into RestClient and fix the remaining coding standards issues.
- Replace README.txt with an up-to-date README.md, crediting the Salesforce Suite module.