bgpwtf/invoice: bump for 2020

This doesn't automatically bump generated numbers - however, new users
will at least have sensible IDs now :).

To bump an existing deployment, you will have to do some mild surgery:

 - edit the touched query to once generate a new, appropriate serial
   (eg. 20001)
 - run and seal one invoice
 - restore original code (now with the new 20000 default)

In the future we should have an RPC override for the new sealed final
UID with some basic sanity checks.

Change-Id: Idd8187618869f6ea76f1b187acfbdd2f1c94005b
1 file changed