If there are two HTLCs with the same preimage, lightningd would always
find the first one. By including the id in the `struct htlc_stub`
it's both faster (normal HTLC lookup) and allows lightningd to detect
that onchaind wants to fail both of them.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Also, we split the more sophisticated json_add helpers to avoid pulling in
everything into lightning-cli, and unify the routines to print struct
short_channel_id (it's ':', not '/' too).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>