chan_pjsip: Fix deadlock when masquerading PJSIP channels.

Performing a directed call pickup resulted in a deadlock when PJSIP
channels were involved.

A masquerade needs to hold onto the channel locks while it swaps channel
information between the two channels involved in the masquerade.  With
PJSIP channels, the fixup routine needed to push a fixup task onto the
PJSIP channel's serializer.  Unfortunately, if the serializer was also
processing a task that needed to lock the channel, you get deadlock.

* Added a new control frame that is used to notify the channels that a
masquerade is about to start and when it has completed.

* Added the ability to query taskprocessors if the current thread is the
taskprocessor thread.

* Added the ability to suspend/unsuspend the PJSIP serializer thread so a
masquerade could fixup the PJSIP channel without using the serializer.

ASTERISK-24356 #close
Reported by: rmudgett

Review: https://reviewboard.asterisk.org/r/4034/
........

Merged revisions 424471 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@424472 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/taskprocessor.h

@@ -246,6 +246,16 @@ int ast_taskprocessor_push_local(struct ast_taskprocessor *tps,
  */
 int ast_taskprocessor_execute(struct ast_taskprocessor *tps);
 
+/*!
+ * \brief Am I the given taskprocessor's current task.
+ * \since 12.7.0
+ *
+ * \param tps Taskprocessor to check.
+ *
+ * \retval non-zero if current thread is the taskprocessor thread.
+ */
+int ast_taskprocessor_is_task(struct ast_taskprocessor *tps);
+
 /*!
  * \brief Return the name of the taskprocessor singleton
  * \since 1.6.1