Since we don't compact the gossmap on the fly (FIXME!) we can
easily surpass 4GB in the gossmap, and 32 bit offsets are not
sufficient.
I'm a bit surprised we don't crash immediately, but we've definitely
seen issues.
Changelog-Fixed: gossipd: crash errors with large gossip_store (>4MB) growth on longer-running nodes.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>