site: Move current_transform, _key_timeout and remote_session_id into a struct

We are going to introduce another set of these for the previous key,
and putting them together in this struct means we will be able to talk
about themm easily.

This patch is very mechanical: we move the three variables into the
new struct, and simply search-and-replace the references.

No functional change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>