Add multi-node log file reading

This handles timestamps, sorting, and merging with data.

For simplicity, we read the log files once per node.  Once benchmarks
show if this is a bad idea, we can fix it.

Change-Id: I445ac5bfc7186bda25cc899602ac8d95a4cb946d
diff --git a/aos/BUILD b/aos/BUILD
index 80ba32a..2ae0fec 100644
--- a/aos/BUILD
+++ b/aos/BUILD
@@ -462,6 +462,7 @@
         "@com_github_google_flatbuffers//:flatbuffers",
         "@com_github_google_glog//:glog",
         "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/types:span",
     ],
 )