Add a linked-pair abstraction to bufferevents.
The new bufferevent_pair abstraction works like a set of buferevent_sockets
connected by a socketpair, except that it doesn't require a socketpair,
and therefore doesn't need to get the kernel involved.
It's also a good way to make sure that deferred callbacks work. It's a good
use case for deferred callbacks: before I implemented them, the recursive
relationship between the evbuffer callback and the read callback would
make the unit tests overflow the stack.
svn:r1152
