An insurance upon life may be made payable:
(a)On the death of the insured.
(b)On his surviving a specified period.
(c)Periodically as long as he lives.
(d)Otherwise contingently on the continuance or determination of life.
(e)Upon such terms and conditions and subject to such restrictions as to revocation by the policyholder and control by beneficiaries as shall have been agreed to in writing by the insurer and the policyholder. If no terms and conditions have been agreed to by the insurer and the policyholder during the insured's lifetime then upon such terms and conditions and subject to such restrictions as may be agreed to in writing by the insurer and the beneficiaries. Any such agreement may be rescinded or amended by the parties thereto without the consent of any beneficiary therein designated unless the rights of any such beneficiary have been expressly declared to be irrevocable. No such agreement hereafter made shall vest in the insurer discretion as to the conditions, time, amount, manner or method of payment. The relationship between the insurer and the policyholder or beneficiaries under any such agreement shall be that of debtor and creditor and the insurer shall not be required to segregate funds so held but shall hold them as a part of its general corporate assets.