Remove ValidationContext::host_timestamp()

[plafer]

We should implement this function ourselves in terms of host_height() and host_consensus_state() (see basecoin-rs), since there are no other valid ways to do it. It should no longer be part of the trait.

[Farhad-Shabani]

This reminded me of another issue I came across in IBC-go (cosmos/ibc-go#3957), which is kind of related to #728 as well. As in both places, we assume that the timestamp necessarily comes from a ConsensusState definition.

IBC-go does things a bit differently when it comes to getting timestamps - they use a method called GetTimestampAtHeight under the ClientState interface. Given that light clients can have access to the ConsensusState store, a client can choose whether to use ConsensusState for getting the timestamp (e.g. here) or not where the Wasm light clients can be an example.

Anyhow, worth evaluating the IBC-go design too, particularly as I see the host_timestamp in our implementation is only called by the ClientState impls.

[Farhad-Shabani]

Closing this issue, as with the experience we gained from recent integration projects, specially considering this spec change, the host_consensus_state is not required anymore. Therefore, the host_timestamp does not necessarily comes from host_consensus_state, though if a host want to go that way, still can.