Fix race condition for CEL LINKEDID_END event

This patch fixes to situations that could cause the CEL LINKEDID_END event to
be missed.

1) During a core stop gracefully, modules are unloaded when ast_active_channels
== 0. The LINKDEDID_END event fires during the channel destructor. This means
that occasionally, the cel_* module will be unloaded before the channel is
destroyed. It seemed generally useful to wait until the refcount of all
channels == 0 before unloading, so I added a channel counter and used it in the
shutdown code.

2) During a masquerade, ast_channel_change_linkedid is called. It calls
ast_cel_check_retire_linkedid which unrefs the linkedid in the linkedids
container in cel.c. It didn't ref the new linkedid. Now it does. 

Review: https://reviewboard.asterisk.org/r/1900/
........

Merged revisions 367292 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 367299 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@367309 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/cel.h

@@ -187,6 +187,15 @@ const char *ast_cel_get_ama_flag_name(enum ast_cel_ama_flag flag);
  */
 void ast_cel_check_retire_linkedid(struct ast_channel *chan);
 
+/*!
+ * \brief Inform CEL that a new linkedid is being used
+ * \since 11
+ *
+ * \retval -1 error
+ * \retval 0 success
+ */
+int ast_cel_linkedid_ref(const char *linkedid);
+
 /*!
  * \brief Create a fake channel from data in a CEL event
  *