thorium: use one multiple-producer ring for outbound messages

There is still a big delay between worker_enqueue_message and
worker_pool_dequeue (no more worker_dequeue_message).

[boisvert@bigmem biosal]$ babeltrace ~/lttng-traces/auto-20141019-163604 \
	| grep "message_number = 99,"|awk '{print $4" "$2}'
thorium_message:actor_send: (+0.003715458)
thorium_message:worker_send: (+0.000001634)
thorium_message:worker_enqueue_message: (+0.000001162)
thorium_message:worker_pool_dequeue: (+0.003283447)
thorium_message:node_send: (+0.000001492)
thorium_message:node_send_system: (+0.000001265)
thorium_message:node_dispatch_message: (+0.000001061)
thorium_message:worker_pool_enqueue: (+0.000001729)
thorium_message:worker_receive: (+0.000000797)
thorium_message:node_send_dispatch: (+0.000000152)
thorium_message:actor_receive: (+0.000002138)

Link: https://github.com/GeneAssembly/biosal/issues/751
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add 'worker_publish_message' tracepoint

The delivery of some messages takes too much time.
This patch adds a tracepoint (and a event probe in the code)
to check the head and the tail of the ring when a worker
publishes a message in the multiple-producer ring.

LTTng trace summary:

thorium_message:actor_send: (+0.005478350)
thorium_message:worker_send: (+0.000001624)
thorium_message:worker_enqueue_message: (+0.000001090)
thorium_message:worker_publish_message: (+0.000001243)
thorium_message:worker_pool_dequeue: (+0.003823578)     <=== This delta is very high.
thorium_message:node_send: (+0.000001549)
thorium_message:node_send_system: (+0.000001131)
thorium_message:transport_send: (+0.000001083)   <=== transport_receive is missing (?)
thorium_message:node_receive: (+0.000153789)
thorium_message:node_dispatch_message: (+0.000001406)
thorium_message:worker_pool_enqueue: (+0.000002151)
thorium_message:worker_receive: (+0.000001539)
thorium_message:actor_receive: (+0.000003002)

Here are some details about event "worker_publish_message":

[17:18:17.373454712] (+0.000001243)
bigmem.biosal thorium_message:worker_publish_message:
{ cpu_id = 59 },
{ message_number = 99, message_action = 18718, message_count = 13,
  message_source_actor = 1001563, message_destination_actor = 1000082,
  ring_size = 552, ring_capacity = 2047, ring_head = 113, ring_tail = 665 }

The problem is that the head (consumer) is far behind the tail (producers).

Normal run:
./scripts/assembly_debugging/spate-saureus-fasta-gz-4x8-load.sh
2m3.919s

Link: https://github.com/GeneAssembly/biosal/issues/751
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>