Add basic support for nodes

This adds the infrastructure and configuration language to describe a
multinode world.  This only checks that if there are multiple nodes
setup, everything is both configured for multiple nodes, and that we are
listening and sending data on the correct node per the configuration.

Change-Id: I658ba05620337a210d677c43e5eb840e05f96051
diff --git a/frc971/wpilib/pdp_fetcher.cc b/frc971/wpilib/pdp_fetcher.cc
index cd17d89..62e0e37 100644
--- a/frc971/wpilib/pdp_fetcher.cc
+++ b/frc971/wpilib/pdp_fetcher.cc
@@ -3,6 +3,7 @@
 #include <chrono>
 
 #include "aos/events/event_loop.h"
+#include "aos/events/shm_event_loop.h"
 #include "aos/logging/logging.h"
 #include "frc971/wpilib/ahal/PowerDistributionPanel.h"
 #include "frc971/wpilib/pdp_values_generated.h"
@@ -12,11 +13,11 @@
 
 namespace chrono = ::std::chrono;
 
-PDPFetcher::PDPFetcher(::aos::EventLoop *event_loop)
+PDPFetcher::PDPFetcher(::aos::ShmEventLoop *event_loop)
     : event_loop_(event_loop),
       pdp_values_sender_(event_loop_->MakeSender<::frc971::PDPValues>("/aos")),
       pdp_(new frc::PowerDistributionPanel()) {
-  event_loop_->set_name("PDPFetcher");
+  event_loop->set_name("PDPFetcher");
 
   // SCHED_OTHER on purpose.
   event_loop_->AddPhasedLoop([this](int iterations) { Loop(iterations); },