Make sure configured cache lifetime option is an integer

In 2bee363d we made sure the configured
cache lifetime _must_ be an integer (using `is_int()`).

Unfortunately the form submits a string even if the value was an
integer and that git stored in the config system as string.

This change:

* converts the submitted value to an integer before storing it
  in the config system
* ensures the configured value is a numeric instead of an integer before
  converting it to integer and using it
* updates already stored numeric strings to integers in the config system