Fix feerate calculation on closing transactions

Fix feerate calculation on closing transactions

This resolves a number of bugs around how we calculate feerates on
closing transactions:

 * We previously calculated the weight wrong both by always
   counting two outputs instead of counting the number of outputs
   that actually exist in the closing transaction and by not
   counting the witness redeemscript.
 * We use assertions to check the calculated weight matches what we
   actually build (with debug_assertions for variable-length sigs).
 * As an additional sanity check, we really should check that the
   transaction had at least min-relay-fee when we were the channel
   initator.