public async Task<bool> AddRefreshToken(RefreshToken token)
        {

            var existingToken = _ctx.RefreshTokens.Where(r => r.Subject == token.Subject && r.ClientId == token.ClientId).SingleOrDefault();

            if (existingToken != null)
            {
                var result = await RemoveRefreshToken(existingToken);
            }

            _ctx.RefreshTokens.Add(token);

            return await _ctx.SaveChangesAsync() > 0;
        }
 public async Task<bool> RemoveRefreshToken(RefreshToken refreshToken)
 {
     _ctx.RefreshTokens.Remove(refreshToken);
     return await _ctx.SaveChangesAsync() > 0;
 }