wallet: cache get_addr_balance
wallet: cache get_addr_balance

notably this makes get_history faster; as 40% of the time is spent in
get_addr_balance (without the cache)

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