Re-enable MMTEL UTs.

Enable one MMTEL UT that can be made to work. (with some workarounds
for a Privacy bug).

Leave one that relies on multiple Privacy invocations disabled,
because it doesn't work.

Add a new MMTEL UT covering CDIV. This currently fails.
Fix dodgy-looking exit-context call.
Pull message handling into separate method.

handle_non_cancel now encapsulates incoming, originating, terminating, and
outgoing handling in a single method.

The initial 100 Trying is not in this method, because it should only happen
once even in recursive MMTEL cases.
Fix MMTEL orig-cdiv handling.

The internal MMTEL AS (in terminating mode) now holds an ODI token (an AsChainLink, actually). This enables it to correctly trigger
orig-cdiv handling when it does a redirect.

Fix the orig-cdiv handling to get the served user from the AsChain we have in our hands, rather than scraping the message for it -
previously this relied on the P-Served-User still being present in the message when the AS returns it, which is not guaranteed,
and now (in the internal case) the P-Served-User header might never have been added!
Fix memory leak: free iFCs when done.
Extra UT to test double-redirection.
Clean up comments.