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/Concerns/HasDatabase.php

@@ -28,7 +28,8 @@ trait HasDatabase
                 }
 
                 if ($key === $this->internalPrefix() . 'db_connection') {
-                    // Remove DB connection because that's not used here
+                    // Remove DB connection because that's not used for the connection *contents*.
+                    // Instead the code uses getInternal('db_connection').
                     continue;
                 }