All the daemons will use a common seed for point derivation, so drag
it out of lightningd/opening.
This also provide a nice struct wrapper to reduce argument count.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This object is basically the embodyment of BOLT #2. Each HTLC already
knows its own state; this moves them between states and keeps them
consistent.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>