Increase precision of locked amounts in shielded token params

[sug0]

In the shielded token param struct,

pub struct ShieldedParams {
    pub max_reward_rate: Dec,
    pub kd_gain_nom: Dec,
    pub kp_gain_nom: Dec,
    pub locked_amount_target: u64,
}

locked amounts are represented as u64 values. We should increase their precision to 256 bits, by switching to a Uint here.

This change would break the genesis files, since Uint values are parsed from strings instead of numbers.