wallet: cache more in get_tx_fee
wallet: cache more in get_tx_fee

closes #4879

vialectrum/vialectrum,pooler/electrum-ltc,spesmilo/electrum,neocogent/electrum,spesmilo/electrum,wakiyamap/electrum-mona,neocogent/electrum,vialectrum/vialectrum,spesmilo/electrum,wakiyamap/electrum-mona,fujicoin/electrum-fjc,pooler/electrum-ltc,pooler/electrum-ltc,neocogent/electrum,spesmilo/electrum,fujicoin/electrum-fjc,wakiyamap/electrum-mona,vialectrum/vialectrum,pooler/electrum-ltc,fujicoin/electrum-fjc,wakiyamap/electrum-mona