The requirements for accepting the remote config are more complex than
a simple min/max value, as various parameters are related. It turns
out that with a few assumptions, we can boil this down to:
1. The valid feerate range.
2. The minimum effective HTLC throughput we want
3. The a maximum delay we'll accept for us to redeem.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>