synchronizer: fix adding duplicate addresses race
synchronizer: fix adding duplicate addresses race

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