Tendermint light client: add consensus state pruning on client update

[plafer]

ibc-go's tendermint light client prunes old consensus states that can no longer be used upon updating the client with a new consensus states. This is a great idea as it keeps the storage requirements of the light client constant over time.

Pruning on every update might be needlessly expensive though; we could even make the point where pruning runs configurable (e.g. every X days, or every Y block, etc).