First part of a comm layer rework

First part of a comm layer rework

* Remove new-style CommWrite, CommFill
* Add in a low-overhead C replacement for comm_read and comm_write
* Remove the difference between comm_write and comm_old_write; everything
  is back to using the same API for now
* create a simple C-based read/write IO callback mechanism, used by
  comm_read and comm_write

Whats left to do:

* convert accept/connect to using the new C-based read/write IO callback
  mechanism (and remove the requirement to allocate/free transient
  structures)
* rework the deferred read manager code somewhat
* then, remove the new-style CommRead code and use comm_read + whatever
  the deferred read manager has become
* remove whats not being used