wallet2: try to split dust sweep txs exponentially
wallet2: try to split dust sweep txs exponentially

This ensures even massive wallets full of dust can sweep.

eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,ranok/bitmonero,ranok/bitmonero,ranok/bitmonero,ranok/bitmonero