When running with IOCP, we are in theory able to use userspace-
allocated buffers to avoid filling up the stingy amount of kernel
space allocated for sockets buffers.

The bufferevent_async implementation in Libevent provides this
ability, in theory.  (There are likely to be remaining bugs).  This
patch adds a new option that, when using IOCP bufferevents, sets
each socket's send and receive buffers to 0, so that we should use
this ability.

When all the bugs are worked out here, if we are right about bug 98,
this might solve or mitigate bug 98.

This option is experimental and will likely require lots of testing
and debugging.