Implement message space gap for DGK decryption

[mihaitodor]

Ideally, we shouldn't try to add u items in the DGK decryption map here, since we don't want to restrict the size of u. Instead, we should implement a mechanism similar to the one in ElGamal, where we leave a gap in the middle of the message space. For this we'll need to add an extra parameter to DGK representing the message space threshold, which can be smaller than u.