Reduce our stated max closing-transaction fee to be the true value

Reduce our stated max closing-transaction fee to be the true value

When communicating the maximum fee we're willing to accept on a
cooperative closing transaction to our peer, we currently tell them
we'll accept `u64::max_value()` if they're the ones who have to pay
it. Spec-wise this is fine - they aren't allowed to try to claim
our balance, and we don't care how much of their own funds they
want to spend on transaction fees.

However, the Eclair folks prefer to check all values on the wire
do not exceed 21 million BTC, which seems like generally good
practice to avoid overflows and such issues. Thus, our close
messages are rejected by Eclair.

Here we simply relax our stated maximum to be the real value - our
counterparty's current balance in satoshis.

Fixes #1071