Dedicated APIs for getting ledger's public key + hash to be used in hash pre-image scheme

[emschwartz]

Right now the /transfers/:id/state endpoint is used for this, which is pretty hacky. A dedicated endpoint would be preferable.

This is also used to get the hash that will be used in the hash pre-image signature scheme. That should also be separated into a different API. Theoretically that is provided by the recipient. Right now the ledger is acting as the recipient's agent. However, even if it continues to do so that behavior should be made more modular and separated out.