network: fix another race in session.subscribe
network: fix another race in session.subscribe

key in session.subscriptions does not imply key in session.cache

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