relayer support for working with pruned nodes

[omritoptix]

we need to understand what are the limitations of the relayer and how much historical state does a node need to keep in order for the relayer to be able to connect to it.

[liorzilp]

Actually, we want to avoid being in an expired state that requires an IBC proposal as happened to us before. So it should be at least the TrustingPeriod. The trusting period default is set to be 85% of the unbonding time, but can't be bigger than the UnbondingPeriod due to protocol assumptions.
Therefore, I recommend having at least the unbonding time.