ServerPool.ListenAndRun -> ServerPool.Listen:

  This func doesn't run a list of servers before listening,
  since this list is always empty and we don't plan on filling it.
  Plus, it was causing a possible datarace by ranging the server slice.
Refine and fix locks in MountMap:

 * MountMap.Delete was only requiring a RLock despite we were deleting
   items from the map
 * Move from RWMutext to Mutex