Tweaks to target refresh; basic UPDATE support

- Target refresh: according to RFC 3261 section 8.2, request processing is
  atomic.  If a request is rejected, all state changed MUST NOT be
  performed.  This now applies to target refresh.
- Basic support for (empty) in-dialog UPDATE requests has been added, for
  use as a session refresh mechanism for a remote session timer endpoint.
  This was quicker and easier than properly supporting reINVITEs, though
  we should also do that in the short term.