Refactor DatabaseConfig, minor DB manager improvements, resolve todos

Notable changes:
- CreateUserWithRLSPolicies: Clarify why we're creating a custom
  DatabaseConfing instance
- HasDatabase: Clarify why we're ignoring tenancy_db_connection
- DatabaseConfig: General refactor, clarify the role of the host conn
- SQLiteDatabaseManager: Handle trailing DIRECTORY_SEPARATOR
  in static::$path
- DisallowSqliteAttach: Don't throw any exceptions, just silently fail
  since the class isn't 100% portable
- Clean up todos that are no longer relevant
- Clean up dead code or comments in some database managers

src/Database/TenantDatabaseManagers/PermissionControlledMySQLDatabaseManager.php

@@ -23,7 +23,6 @@ class PermissionControlledMySQLDatabaseManager extends MySQLDatabaseManager impl
     {
         $database = $databaseConfig->getName();
         $username = $databaseConfig->getUsername();
-        $hostname = $databaseConfig->connection()['host'];
         $password = $databaseConfig->getPassword();
 
         $this->connection()->statement("CREATE USER `{$username}`@`%` IDENTIFIED BY '{$password}'");