Switch magic timestamp channel to RemoteMessage

In order to track reboots and such, we really need to communicate more
information from the message_bridge to the logger.  It is a shame to
have to modify the MessageHeader to do this, even though it would be
nice.  Switch the remote timestamp channels over to a new RemoteMessage
channel instead, and add code to rename the channel when replaying.

There are no known log files with a MessageHeader actually logged, so
most of this should be pretty safe.  I've tested this on an old log file
by hand.

Change-Id: If81b31869b95040151d833d20ec3eb8623ab1cd4
diff --git a/aos/flatbuffer_introspection_test.cc b/aos/flatbuffer_introspection_test.cc
index cd005ce..1119b3b 100644
--- a/aos/flatbuffer_introspection_test.cc
+++ b/aos/flatbuffer_introspection_test.cc
@@ -10,13 +10,13 @@
 class FlatbufferIntrospectionTest : public ::testing::Test {
  public:
   FlatbufferIntrospectionTest()
-      : schema_data_(
-            util::ReadFileToStringOrDie("aos/json_to_flatbuffer.bfbs")) {
+      : schema_data_(FileToFlatbuffer<reflection::Schema>(
+            "aos/json_to_flatbuffer.bfbs")) {
     schema_ = reflection::GetSchema(schema_data_.span().data());
   }
 
  protected:
-  FlatbufferString<reflection::Schema> schema_data_;
+  FlatbufferVector<reflection::Schema> schema_data_;
   const reflection::Schema *schema_;
 };