Implement KeysCollectionDocument

{

    _id: <int>,

    purpose: 'signLogicalTime',

    key: <20 byte key generated with secure PRNG in BinData>,

    expiresAt: <LogicalTime, currently Timestamp format>

}

class KeysCollectionDocument {

public:

    KeysCollectionDocument(long long keyId, std::string purpose, TimeProofService::Key key, LogicalTime expiresAt);

    static StatusWith<TypeKeys> fromBSON(const BSONObj& doc);

    BSONObj toBSON() const;

    // getter methods

};