Tendermint34Client maxAge

[codehans]

On the Kujira dapps we use a community sourced set of RPC clients to provide chain connections for users, in order to promote redundancy and decentralisation. We use a Promise.any approach to selecting an RPC provider when a UI initialises, in order to pick one with low latency to the user. Sometimes some of these clients fall behind yet still have a fast response time, particularly when you're accessing from a geographically underserved location. This results in stale data, account sequence mismatches, etc.

This PR adds an optional maxAge parameter to the connect function of Tendermint34Client, and throws the connection if the reported latest_block_time is stale.

@webmaster128 - will sort out tests etc for this if you feel it's a useful addition. As it stands, we're pushing for decentralisation of infrastructure, but this is making the dApps unusable for some connections.

[webmaster128]

Very interesting idea and sane implementation. I like not making a single Tendermint client responsible for the balancing but having that at a higher level.