Geth --historymode flag

[fjl]

We need a way to configure history sync in geth, so that users can enabled the pruned mode. Eventually, Geth will default to pruned history by default.

This should be similar to --syncmode and --gcmode, i.e. a union type in eth/ethconfig. For simplicity, we should not allow configuring an arbitrary pruning block for this. The two options will be:

type HistoryMode int

const (
     AllHistory HistoryMode = iota
     PrunedHistory
)