|Returns the user ID associated with a connection. The userid may be used by the DataScript in a number of ways, such as evaluating a user against a white or blacklist. This field is also used in the client logs to populate the searchable UserID field. This field is populated via the following methods, prioritized in the following order:
|String of the user ID, ‘nil’ if empty.
|If HTTP header 'username' exists, insert its value into the UserID field, else set the field to none.