[csma] Add full-duplex mode for csma channels

I realize this has been brought up a few times before, but it's never been landed. Figured I'd take a stab at resurrecting it since I need the feature anyway :)

Relatively similar to what was proposed in https://codereview.appspot.com/187880044/ / https://www.nsnam.org/bugzilla/show_bug.cgi?id=2354 which I found after having written much of this (oops).

I included a test case for full-duplex operation but feel uncomfortable about the lack of tests for half-duplex -- any recommendations on what to add there?