Remove duplicate code in SessionBase, SessionManager and Session

Patch removes duplicity in code and corrects session documentation

Accidentally missed this and filed #6081. This is more comprehensive, but #6081 has already been checked in.

Goes beyond #6081.

Also, there should be clear instructions how to clear all session data (as opposed to deleting individual keys).

request.session.clear() seems to do it, it should be documented (I assume this is correct looking at the code -- looks like the the corresponding row will be deleted from session table if session dict is empty) or an explicit, documented method for clearing the session should be added.

Perhaps a note like the following should be added:

Use request.session.clear() if you need to clear all session data. The corresponding session row will be automatically removed from the session table when session dictionary is empty.

Test cases should be added too, in the lines of the following:

>>> request = HttpRequest
>>> # attach session to HttpRequest
>>> request.session["foo"] = "bar"
>>> request.session.clear()
>>> request.session["foo"]
Traceback (most recent call last):
    ...
KeyError: 'foo'

See e.g. http://beaker.groovie.org/beaker/browser/beaker/session.py#L174 for session invalidation that really should be present in Django.

Anonymous, please don't hijack tickets for issues of your own. Your item has nothing to do with this ticket. If you want extra functionality added, read contributing.txt for how to propose it (discussion on django-dev and then maybe a ticket).