This commit saves the RPC request used to construct the `Parameters` on
disk. Since it's a proto message, an easy way to read/write it is to
rely on the proto marshal/unmarshal methods. A side effect is that
migration also becomes easy as proto message have its own internal
mechanism to keep track of the compatibility.
This commit refactors the method `manager.SetParameters` to take a
`SetLiquidityParamsRequest`. As we'll see in the following commit, this
will enable us saving the params to disk more easily.
