fix a race condition in synchronizer
fix a race condition in synchronizer

wallet.synchronizer gets assigned a newly constructed Synchronizer instance.
Synchronizer in tx_response refers to the value of wallet.synchronizer.
If the wallet has a missing txn, there could be a race condition that synchronizer asks for a txn and we get the callback from the network WHILE the constructor is still running, in which case wallet.synchronizer would still be None and we would consider the callback "orphan", and the wallet would get "stuck" synchronizing.

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