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.