logger: Added logger_queue_limit to the configuration options.

All log messages go to a queue serviced by a single thread
which does all the IO.  This setting controls how big that
queue can get (and therefore how much memory is allocated)
before new messages are discarded. The default is 1000.
Should something go bezerk and log tons of messages in a tight
loop, this will prevent memory escalation.

When the limit is reached, a WARNING is logged to that effect
and messages are discarded until the queue is empty again.  At
that time another WARNING will be logged with the count of
discarded messages.  There's no "low water mark" for this queue
because the logger thread empties the entire queue and processes it
in 1 batch before going back and waiting on the queue again.
Implementing a low water mark would mean additional locking as
the thread processes each message and it's not worth it.

A "test" was added to test_logger.c but since the outcome is
non-deterministic, it's really just a cli command, not a unit
test.

Change-Id: Ib4520c95e1ca5325dbf584c7989ce391649836d1

include/asterisk/logger.h

@@ -525,6 +525,22 @@ void ast_verb_console_set(int verb_level);
  */
 int ast_is_logger_initialized(void);
 
+/*!
+ * \brief Set the maximum number of messages allowed in the processing queue
+ *
+ * \param queue_limit
+ *
+ * \return Nothing
+ */
+void ast_logger_set_queue_limit(int queue_limit);
+
+/*!
+ * \brief Get the maximum number of messages allowed in the processing queue
+ *
+ * \return Queue limit
+ */
+int ast_logger_get_queue_limit(void);
+
 #if defined(__cplusplus) || defined(c_plusplus)
 }
 #endif