Running on a shared "wan_server" thread? Use non-blocking sockets. Still WIP -- specifically, this breaks large writes.
 * see new TODO in BufferedConnectionWriter.cpp
 * see new TODOs in WriteActionSender.cpp
Better implementation/interface for BufferedConnectionWriter.
 Now handles nonblocking and blocking writes concurrently and correctly.
 Easier to fix it once it exists...
 Also gave StartupTest a way to suppress SIGPIPE-splosions.