o Major feature (scheduler, channel):
    - Introducing the KIST scheduler which stands for Kernel Informed Socket
      Transport. It is only available on Linux systems. This comes from a
      researched and published paper you can find here:

      http://www.robgjansen.com/publications/kist-sec2014.pdf
      https://arxiv.org/abs/1709.01044

      This is also a major refactoring of the entire scheduler subsystem in
      order for it to be more modular and thus much more easier to add more
      scheduler type later. The current scheduler has been named "Vanilla" but
      we favor KIST if available in this version.

      A new torrc option has been added and named "Schedulers type1,type2,..."
      which allows a user to select which scheduler type it wants tor to use.
      It is also possible to change it at runtime. It is an ordered list by
      priority. KIST might not be available on all platforms so there is a
      fallback to "KISTLite" that uses the same mechanisms but without the
      kernel support.

      The current default values are: Schedulers KIST,KISTLite,Vanilla.

      Closes ticket 12541.