res_parking: Fix crash due to race condition when unloading.

res_parking: Fix crash due to race condition when unloading.

There is currently a race condition when unloading the res_parking
module. Depending on the will of the universe the subscription
invocation may occur AFTER the module is unloaded. This is because
the module does NOT use stasis_unsubscribe_and_join when terminating
the subscription. It merely uses stasis_unsubscribe.

This change makes it use stasis_unsubscribe_and_join which is documented
for usage in this exact scenario.

AST-1520 #close

Review: https://reviewboard.asterisk.org/r/4375/
........

Merged revisions 431114 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/13.1@431116 65c4cc65-6c06-0410-ace0-fbb531ad65f3