Update logic to choose yggdrasil send out fund

during some high volume test , if the thornode choose a yggdrasil to send out fund, however the yggdrasil still have multiple blocks of txout not send out yet, it might cause thornode to choose a yggdrasil that doesn't have enough fund to send

This PR will go back 20 blocks, and take the txout for the last 20 blocks into consider when it try to choose a yggdrasil to send out fund.

Which is more stable and faster when running a lot swaps during my test.