Update IsInitialBlockDownloadInit() within ActivateBestChainStep
Update IsInitialBlockDownloadInit() within ActivateBestChainStep

This fixes a problem with ZMQ after the new atomic
IsInitialBlockDownload() was merged.  We have to make sure the value
is updated after each block is found.

cddjr/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Bitcoin-com/BUcash,BTCfork/hardfork_prototype_1_mvf-bu,krzysztofwos/BitcoinUnlimited,Justaphf/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,marcusdiaz/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,krzysztofwos/BitcoinUnlimited,cddjr/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Bitcoin-com/BUcash,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,BitcoinUnlimited/BitcoinUnlimited,cddjr/BitcoinUnlimited,Bitcoin-com/BUcash,cddjr/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,krzysztofwos/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,Justaphf/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Justaphf/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,Bitcoin-com/BUcash,cddjr/BitcoinUnlimited,cddjr/BitcoinUnlimited,Bitcoin-com/BUcash,BitcoinUnlimited/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,BTCfork/hardfork_prototype_1_mvf-bu,marcusdiaz/BitcoinUnlimited,Bitcoin-com/BUcash