Add storage gap to RewardsStreamerMP to reserve storage slots for future upgrades

[0x-r4bbit]

The RewardsStreamerMP contract (aka stake manager) is upgradeable.

To reduce the likelihood of storage collisions in the future, for cases when new fields are added (like how it happened in #123 ), it's a recommended practice to introduce a storage gap.

The idea is to add a static array declaration to the contract, which reserves the same amount of storage slots as the size of the array.

contract ExampleV1 {
    uint256 public somProp;
    uint256 public someOtherProp;
}


contract ExampleV2 {
    uint256 public somProp;
    address someAddress; // <- oops collision!
    uint256 public someOtherProp;
}

// storage gap version
contract ExampleV1 {
    uint256 public somProp;
    uint256 public someOtherProp;
    uint256[50] private __gap; // <-- 50 reserved additional slots
}

// now adding an `address` field in between is fine:

contract ExampleV2 {
    uint256 public somProp;
    uint256 public someOtherProp;
    address someAddress;   
    uint256[49] private __gap; // <-- reduce gap by 1 slot!
}