Retrieve a value from the Zotonic visitor record.
The visitor record’s value for the key
argument is returned:
http://localhost:8000/api/base/persistent_get?key=foo
might return:
{ "key": "foo", "value": null }
If the value is undefined; or
{ "key": "foo", "value": "bar" }
If the value was set.
.. seealso:: :ref:`service-persistent_set`