InvoiceRequest metadata and payer id derivation

InvoiceRequest metadata and payer id derivation

Add support for deriving a transient payer id for each InvoiceRequest
from an ExpandedKey and a nonce. This facilitates payer privacy by not
tying any InvoiceRequest to any other nor to the payer's node id.

Additionally, support stateless Invoice verification by setting payer
metadata using an HMAC over the nonce and the remaining TLV records,
which will be later verified when receiving an Invoice response.