CEL: Protect data structures during reload and shutdown.

The CEL data structures need to be protected during a configuration reload
and shutdown.  Asterisk crashed during a shutdown because CEL events were
still in flight and the CEL data structures were already destroyed.

* Protected the cel_backends, cel_dialstatus_store, and cel_linkedids ao2
containers with a global ao2 object wrapper.

* Added NULL checks before use of the cel_backends, cel_dialstatus_store,
and cel_linkedids ao2 containers in case the CEL module is already
shutdown.

* Fixed overloading of the cel_linkedids held objects reference count.
During shutdown any held objects would be leaked.

* Fixed memory leak of cel_linkedids held objects if the LINKEDID_END is
not being tracked.  The objects in the cel_linkedids container were not
removed if the LINKEDID_END event is not used.

* Added access protection to the cel_backends container during the CLI
"cel show status" command.

* Made cel_backends, cel_dialstatus_store, and cel_linkedids use the
standard ao2 callback templates for the hash and cmp functions.

* Eliminated unnecessary uses of RAII_VAR().

* Made ast_cel_engine_init() cleanup alocated resources on failure.

(closes issue AST-1253)
Reported by: Guenther Kelleter

Review: https://reviewboard.asterisk.org/r/3128/
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Merged revisions 406417 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 406418 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@406465 f38db490-d61c-443f-a65b-d21fe96a405b