Clarify SetNewParams() in x/stake

[mossid]

SetNewParams() in x/stake sets all parameter from params struct. This will be used only for initializing all parameters in testing. We should define SetXXX()s for individual parameters and use it for production code.

Also see: #1772 conversation.

[ValarDragon]

If we have code like this for tests, lets make it visually obvious. e.g. instead of
SetNewParams, instead do TestingSetNewParams or SetNewParamsForTesting.

[rigelrozanski]

The params should probably be moved to the global params store anyways - not sure that it will matter after then