Enable disabled fallbackfee in code to avoid "Fee estimation failed" errors

Version 0.18.1.9

As I understand it the fallbackfee that is set within code is actually ignored (inherited from Bitcoin Core). I think this was done to avoid users getting txs rejected if it was set too low.

In Elements it takes a long time for a node to calculate the needed fee rate due to having much less txs than bitcoin. It doesn't matter if they are using QT because the fee rate is set in the GUI. For users who use elements-cli or scripts using RPC it means they can wait hours for the fees to be calculated.

You can specify `fallbackfee=0.00000100` in `elements.conf` to get around waiting but it would be good if the default in elements code worked. Many users start their elements node to perform some task then shut it down and this can result in fee calculation issues.

ソースファイルもテストも指定されていません。まず、elements-cli と RPC 呼び出しで fallbackfee がどのように扱われているかを追跡し、elements.conf の設定可能な値と比較して、手数料の推定を完了できない場合の動作を確認してください。コードのデフォルト値によって、手動設定を必要とせずに報告されたエラーが防止されれば完了です。

索引モデルが issue の本文から書いたものです。