This implements the proposal to simply use timestamp as "all", "none"
or "stream". There's also a rough spec draft which I will post soon.
This *also* removes the last place where we would sometimes sweep the
entire gossip_store looking for their given timestamps.
We could also get rid of the actual timestamp filtering logic in
gossip_store_next if we want to, as it's now basically unused.
Changelog-Changed: Protocol: Simplify gossip_timestamp_filter handling to "all", "none" or "recent" instead of exact timestamp.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>