If there's a rate-card for liquidity, we don't know about it until
after startup (the plugin *should* call us at init to tell us what their
current rates are)
I was seeing some accidental pruning under load / Travis, and in
particular we stopped accepting channel_updates because they were 103
seconds old. But making it too long makes the prune test untenable,
so restore a separate flag that this test can use.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
