[RPC] report PoX anchor block once discovered when in Nakamoto

[jcnelson]

Right now, the /v2/info handler reports the cached anchor block information in the PeerNetwork instance. This in turn is loaded via a SortitionHandleConn, which reads the selected anchor block from the MARF.

The trouble is, this record of the anchor block information is only stored at the start of the relevant reward cycle. While this was okay in epoch 2.x, in Nakamoto we know the anchor block after the second sortition in the prepare phase.

The PeerNetwork code should load the anchor block from the Nakamoto chainstate when caching anchor block data for a Nakamoto reward cycle.