Secondary session index update fails when a fixed NameID is used

The session manager has a flaw in the indexBySPSession routine that maintains the backwards index from SP+NameID -> session ID for SAML logout.

If the same SP + NameID is indexed against the same session a second time, the code is broken and it falls into a retry loop and retries the operation 10 times before giving up. The actual impact is just overhead because the session's already indexed, but it's noisy and confusing in the log, plus wastes I/O.