Use init_cache_size instead of cache_size when clearing session store.

Just make it clear that we should always be using init_cache_size. In
practice this commit doesn't actually affect the program, it just
clarifies the code.
Fix access of uninitialized data when logging out.

Since the session data for an unused session isn't initialized,
we may end up reading uninitialized data, leading to a segmentation
fault.
Add support for AssertionConsumerServiceURL

Apperently some IdPs like PicketLink insist they reaaaally want to see
AssertionConsumerServiceURL as part of the Authentication Request.

Provide it if allowed by the SP metadata.

Signed-off-by: Simo Sorce <simo@redhat.com>