ICS2: Add getTimestampAtHeight function to client state interface

[colin-axner]

In ibc-go we are rewriting the getTimestampAtHeight functionality. Currently it is housed in ICS 003, it's functionality is to obtain the consensus state at the proof height and check its timestamp value. The problem with this is the solo machine doesn't store consensus states at each height it has had a consensus state. This is because the solo machine only maintains the latest consensus state.

I propose the spec to do the same changes (or devise another way to get timeouts to work with the solo machine). This would be appreciated as some clients may have not so apparent requirements to fulfill this function. For example, the solo machine should return the timestamp of the latest consensus state. If the height passed in is > solomachine.Sequence + 1, it should return an error.