We use the high bit of the length field: this way we can still check
that the checksums are valid on deleted fields.
Once this is done, serially reading the gossip_store file will result
in a complete, ordered, minimal gossip broadcast. Also, the horrible
corner case where we might try to delete things from the store during
load time is completely gone: we only load non-deleted things.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>