This commit swaps out golang/protobuf/jsonpb for a custom variant that
by default prints byte slices as hex, which is more useful for our
setting. Some existing wrapper structs are removed as they can now be
printed directly with the new jsonpb.
!!! NOTE !!!
This commit introduces a breaking change to lncli listinvoices since
payment hashes and preimages will now be printed in hex instead of
base64.
