Add timer for periodic Namecoin name updates.

Add a timer that periodically calls a slot to be used for updates of
Namecoin name registration processes.  It doesn't yet do that, though.
Partial name updates in timer.

In the Namecoin update timer call, partially perform name updates.  It
now issues name_firstupdate's as necessary, but doesn't yet remove
finished name registration processes and also doesn't yet do the final
name_update to send names to the necessary address.