ledger: faster sign_transaction startup
ledger: faster sign_transaction startup

Only call Ledger_KeyStore.get_client_electrum() once,
as it runs DeviceMgr.scan_devices(), which is slow.

spesmilo/electrum,vialectrum/vialectrum,wakiyamap/electrum-mona,spesmilo/electrum,pooler/electrum-ltc,spesmilo/electrum,wakiyamap/electrum-mona,wakiyamap/electrum-mona,vialectrum/vialectrum,vialectrum/vialectrum,pooler/electrum-ltc,pooler/electrum-ltc,pooler/electrum-ltc,wakiyamap/electrum-mona,spesmilo/electrum