Simplify MemcacheStore at the cost of ease-of-use.

If MemcacheStore is used to store objects which are changed, then they
should be added to MemcacheStore again after they have changed.

This change also fixes a potential problem when both the MemcacheStore class
and the Session class uses register_shutdown_function. Earlier one could get
a situation where MemcacheStore-objects's shutdown-function was run first,
followed by the Session-object's. In that case the changes to the Session-
object wouldn't be saved.


git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@572 44740490-163a-0410-bde0-09ae8108e29a