Add GetTimestampAtHeight to client state interface

[colin-axner]

Summary

The solo machine cannot currently process timeouts because it does not have a consensus state associated with the timeout height until after processing the timeout. Core IBC assumes a consensus state exists in state for the timeout height, but solo machines stores the consensus state in the client state.

We should add a function GetTimestampAtHeight to the client state function which returns the timestamp at a given consensus state height.

Splitting PR recommendation

PR 1

Add the GetTimetstampAtHeight to each client state + unit tests

• [x] 06-solomachine
• [x] 07-tendermint
• [x] 09-localhost

PR 2

Modify connection keeper GetTimestampAtHeight to use the client state interface function. See if it is possible to write a test for solo machines to send a successful timeout msg

For Admin Use

• [x] Not duplicate issue
• [x] Appropriate labels applied
• [x] Appropriate contributors tagged/assigned

[crodriguezvega]

I have asked today if it's possible to merge Colin's PR to move forward with PR1 of this issue.