Back out the session_start and session_end hooks feature.

Back out the session_start and session_end hooks feature.

It's become apparent during testing that there are problems with at
least the testing regime. I don't think we should have it without a
working test regime, and the difficulties might indicate implementation
problems anyway, so I'm backing out the whole thing until that's sorted
out.

This reverts commits 7459484 9989f92 cd8ce3a