[#3684] Do polyfill for Session component as well

- SessionArrayStorage and Container were both doing version-specific conditional
  declarations; moved these into a compat layer as was done in Stdlib, and
  updated the composer.json for each of the component and framework.
- Currently, the SessionConfigTest suite is not running for me, but appears to
  be a random autoloading error from running in isolated processes; will see how
  the Travis-CI run looks.