[wanproxy/wanproxy] bdbefa: Further clean up and simplify SocketUinet.

[Juli Mallett]

  Branch: refs/heads/master
  Home:   https://github.com/wanproxy/wanproxy
  Commit: bdbefa5137a9290fd39e827f9a4a9dec80c77d0b
      https://github.com/wanproxy/wanproxy/commit/bdbefa5137a9290fd39e827f9a4a9dec80c77d0b
  Author: Juli Mallett <juli at clockworksquid.com>
  Date:   2015-04-16 (Thu, 16 Apr 2015)

  Changed paths:
    M io/socket/socket_uinet.cc
    M io/socket/socket_uinet.h

  Log Message:
  -----------
  Further clean up and simplify SocketUinet.

This cleans up some of the locking and scheduling, and also narrows
the period in which upcall handlers are registered to the period
where we need them.

This also eliminates the need for upcallprep routines, which was
discussed previously with and recommended by @pkelsey.