Writes an approval request object.

def WriteApprovalRequest(self, approval_request):
    """Writes an approval request object."""
    approvals = self.approvals_by_username.setdefault(
        approval_request.requestor_username, {})

    approval_id = str(os.urandom(16).encode("hex"))
    cloned_request = approval_request.Copy()
    cloned_request.timestamp = rdfvalue.RDFDatetime.Now()
    cloned_request.approval_id = approval_id
    approvals[approval_id] = cloned_request

    return approval_id