Add SourceNodeIndex to compute the source node per channel

We keep doing this type of thing.  Time to go write a function.  Doing a
bunch of string compares is slow, so we should really cache it up front.

It feels like it might be getting close to time to try to write a class
which will let us compute and cache all these things reasonably
efficiently and move logger and others over to it.  Let's start with a
function to compute the things we care about.

Change-Id: I4b788b1395656e8e6e6c3c7eb28f4d410080c636
Signed-off-by: Austin Schuh <austin.schuh@bluerivertech.com>
diff --git a/aos/configuration.cc b/aos/configuration.cc
index c2e42c9..d794a37 100644
--- a/aos/configuration.cc
+++ b/aos/configuration.cc
@@ -1261,5 +1261,18 @@
   return nullptr;
 }
 
+std::vector<size_t> SourceNodeIndex(const Configuration *config) {
+  CHECK(config->has_channels());
+  std::vector<size_t> result;
+  result.resize(config->channels()->size(), 0u);
+  if (MultiNode(config)) {
+    for (size_t i = 0; i < config->channels()->size(); ++i) {
+      result[i] = GetNodeIndex(
+          config, config->channels()->Get(i)->source_node()->string_view());
+    }
+  }
+  return result;
+}
+
 }  // namespace configuration
 }  // namespace aos