- Introduce the field `CustomRecords` to the type `UpdateAddHtlc`.
- Encode and decode the new field into the `ExtraData` field of
the `update_add_htlc` wire message.
This commit does away with all the old manual message equality tests
and replace it with a single property-based test that uses the
testing/quick package. This test uses a single scenario which MUST hold
for all the messages type and all possible messages generated for those
types. As a result we are able to do away with all the prior manually
generated test data as the fuzzer to scan the input space looking for a
message that violates the scenario.
