Change how nodes fetch history from network

[jedmccaleb]

As a followon to #79 we would like to transmit history over the network primarily as a sequence of ledger-header / transaction-sequence blocks, rather than set-reconciliation of SHAMaps. The starting point of history replay should be either the genesis ledger or an account-state snapshot (i.e. a set of most-recent temporal buckets of account-state, see #118) but from then on, a receiving node should receive history as a transaction stream, and reconstruct its own account-state snapshots (at whatever resolution it likes) deterministically, by history replay.