Make sweep work for less than 500 outputs

[sondreb]

• This fixes an issue where the sweep would only work if there was more than 500 outputs.
• The "currentOutputCount" is not really used and is reset for every single UTXOs discovered and individual transactions is generated for each UTXOs, they are not combined into a single transaction.
• The sweep might need some refactoring, and the FallbackFee might need to be increased a little bit to work properly and not generate transactions with too low fee during sweep.

[sondreb]

I think the original thinking behind this sweep code was to grab all UTXOs and generate a single transaction, but it just doesn't do that right now so this is a tiny fix to make it actually work.

[zeptin]

No, the purpose of the < was to keep adding UTXOs to the builder until the transaction has that many inputs, as a rough heuristic for the maximum possible number. A more robust but expensive approach is to estimate the transaction size after each addition, as different types of transactions have differing scriptSig sizes.

Once a large enough transaction is reached the remainder of the loop executes. You thus get a list of transactions with 500 inputs at the end. So I think more of a refactor would be needed in order to handle the case where there aren't at least 500 inputs. Probably the latter part of the loop should be moved to a method, and it should be executed once more once the loop terminates to make a final transaction with the remaining UTXOs.

[sondreb]

So this small PR will fix it so that it accepts a sweep, but there is more to be done in the implementation I think. I might do some more improvements and make another PR, unless someone else does it. Additionally I had to increase the FallbackFee a little bit to ensure it used enough fee to be approved by the policy. Fee should probably be added as an parameter to the API.

[sondreb]

Made an update to the sweep, please review =)