hammer2 - cluster / libdmsg circuit work

hammer2 - cluster / libdmsg circuit work

* Major work on the virtual circuit code.  Note there are still some thread
  races.

* Major work on the spanning tree code to allow for future symmetric
  pathing support.

* fs_label is now part of the RB tree compare, along with pfs_fsid.
  (e.g. the serial number for BLK protocol services).

* Rip out the 'router' structure, replace with the 'circuit' structure.
  Embed circuit0 in the iocom.