Timestamp and duration units should be specified in English specs

[soareschen]

There are a number of places in the English specs that use u64 as timestamp or duration fields, but with the unit unspecified. One would have to look into the protobuf definitions and the Go code in ibc-go to figure out that the proper unit is in nanoseconds. This can easily cause misinterpretation, and cause the field to be processed in incorrect units such as seconds. (https://github.com/informalsystems/ibc-rs/issues/927)

We should at least clarify in the specs that the relevant u64 fields are in nanoseconds unit. Or if breaking change is possible, we should instead follow Tendermint and use Google.Protobuf.WellKnownTypes.Timestamp and Google.Protobuf.WellKnownTypes.Duration to encode timestamp and duration fields.