Make expires_at optional

The issued_at value is on every token, and will be checked on
every event against the issued_before field.  expires_at
is not going to be mutually exclusive with issued_before:
issued_before will be specified and checked for all tokens, including
those revoked by user and expires_at.  issued_before is
mandatory, and expires_at is optional.

Change-Id: I3e8bcc469ccc7476d517a8f4bd6574ba99dba1f2
Merge "Make expires_at optional"