Clean up resources in features on exit

This patch cleans up two things features:
* It properly unregisters the CLI commands that features registered
* It cancels and performs a pthread_join on the created parking thread. This
  not only properly joins a non-detached thread, but also prevents disposing
  of the parking lots prior to the parking thread completely exiting.

(closes issue ASTERISK-21407)
Reported by: Corey Farrell
patches:
  features_shutdown-r2.patch uploaded by Corey Farrell (License 5909)
........

Merged revisions 386641 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@386642 65c4cc65-6c06-0410-ace0-fbb531ad65f3