Rename %o -> %data in status_proc(); declare %data sooner

Follow-up to 6dabd60 to handle the var rename in status_proc().

%data is now declared with the rest of the common variables.
README update for partial-update API changes in d77808d
Support adding a missing session entry during updates

It is possible for the initial client-connect event to be missing if the
operation failed due to a temporary system or network failure.

A new --update-create flag will enable connect() to insert the missing
session entry.