Ticket 47451 - Need to unregister tasks created by plugins
Bug Description: Tasks created by plugins are not unregistered when a plugin
is stopped or deleted. Repeated stopping/starting a plugin
that registers tasks will corrupt the dse callback linked list
and will crash the server if a task is invoked.
Fix Description: Create a plugin task unregister function, and call it in the
clsoe functions of plugins that register functions.
Reviewed by: nhosoi(Thanks!)
(cherry picked from commit 005c4c9be360a7ebba38b61f934ace94224eef3f)