This commit was previously split into the following parts to ease
review:
- 2d746f68: replace imports
- 4008f0fd: use ecdsa.Signature
- 849e33d1: remove btcec.S256()
- b8f6ebbd: use v2 library correctly
- fa80bca9: bump go modules
With the introduction of additional payload fields for mpp, it becomes
a necessity to have their values available in the on-chain resolution
flow. The incoming contest resolver notifies the invoice registry of the
arrival of a payment and needs to supply all parameters for the registry
to validate the htlc.
