Use timed signer to check for expiration of session data. This is to
fix ticket django#18194. The sessions based on file backend otherwise do not
expire, as far as the server is concerned.

Signed-off-by: Rohan Jain <[email protected]>

Conflicts:
	django/contrib/sessions/backends/base.py

Signed-off-by: Rohan Jain <[email protected]>

The existing sessions, which were not signed with the signing
framework is handled with the older decoding method.
Mark the session as modified so that it uses the new encoding method
for storing the data.

Signed-off-by: Rohan Jain <[email protected]>

Conflicts:
	django/contrib/sessions/backends/base.py

Don't enable compatibility with older mechanism by default as it
compromises with the security benefits of introducing signing
framework.

Signed-off-by: Rohan Jain <[email protected]>

Signed-off-by: Rohan Jain <[email protected]>