[mms] Fix destroying session data when the null session driver is used.

You can't destroy a session that isn't active. Use session_unset() instead
phpdoc
Abstract main session class so extended null class doesn't need to extend much of anything

Even before this change, much of the null class was already duplicative
and unncessary
Fix logic
Merge branch 'master' into imp_6_1

Conflicts:
	imp/lib/Compose.php