calls to store_update_{time,height} should not be done in client handlers

[plafer]

Should be called in tendermint's implementation of ClientState.initialise(), as in ibc-go. We currently call it in the CreateClient handler and the UpdateClient handler.

[plafer]

Also, the "processed time" is the block time of the last header. Instead of storing it as a separate value, couldn't we instead retrieve it from the last stored ConsensusState (which might require extra bookkeeping of its own, such as the height of the last update)?