Positions answer for the live chain, not yesterday

Orion answers from the network as it stands now rather than from where recorded history left off. A position read reflects the chain within seconds of a ledger closing.

• Every response reports ledger_lag_seconds — the time since the served ledger’s recorded close time, null when that close time wasn’t recorded — alongside the last indexed ledger, so how current an answer is, is something you can read rather than assume.