If you get the wrong hsm_secret, your node_id will change, and
peers won't know who you are, bitcoind will reject your transaction
signatures, and other madness.
Catch this as soon as it happens, by storing our node_id in the db.
Suggested-by: @cdecker, @fiatjaf
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: Config: `lightningd` will refuse to start with the wrong node_id (i.e. hsm_secret changes).