Transport: Have a separate channel for recovery
Have a separate channel for recovery, so it won't overflow the "low" channel which is also used for bulk indexing.

Also, rename the channel names to be more descriptive. Change low to bulk (for bulk based operations, currently just bulk indexing), med to reg (for "regular" operations), and high to state (for state based communication). The new channel for recovery will be named recovery, and the ping channel will remain the same.

closes #3954