I noticed that our subtables were not being cleaned, despite being "ON
DELETE CASCADE". This is because foreign keys were not enabled, but
then I got foreign key errors: rowid cannot be a foreign key anyway!
So create a real "rowid" column. We want "ON DELETE CASCADE" for
nodes and channels (and other tables in future) where we update
partially.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>