Improve IBD when pruning
Improve IBD when pruning

There was too much constant flushing of the dbcache which was slowing
down IBD performance when pruning. When we remove this flushing and
let flushing happen normally (when the cache is full), then IBD
perfomance is the same whether the node is a pruning node or a network
node.

BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited