Fix pruning when syncing a chain for the first time
Fix pruning when syncing a chain for the first time

When  syncing a chain with pruning enabled there are
at times new blocks arriving which make the nLastHeight equal
to the tip of the blockchain, however this prevents block files
from being removed during pruning.  By not downloading new blocks
until the chain no longer in IsInitialDownload() the issue is
prevented.

Unfortunately this workaround also requires that we still allow
all blocks to be downloaded on regtest.  While the fix works fine
it's never a great idea to test essentially different code
from what is released on mainnet.  And while there  are a handful of these
cases in the code base providing a precedenche,  it would still be
good to find a more broad based fix for this issue.

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