Setup the message bridge key sharing services

This implements 2 new services that will be used to share the active
SCTP authentication key. For context, if SCTP authentication is wanted,
then we will need a way to securely distribute a shared key across every
node. We use gRPC to distribute the key.

* message_bridge_auth_server

This service should only run in one node. It generates a 128-bit
random-key during initialization. It sets up the gRPC service using
mutual-TLS authentication.

* message_bridge_auth_client

This service will run in every node. It listens for requests
in /aos aos.message_bridge.SctpConfigRequest and requests the
active key from the gRPC server which gets propagated into /aos
aos.message_bridge.SctpConfig. message_bridge reads this value and sets
the authentication key (previous change in relation).

These have some additional side-effects:
* This change also forces us to bring in an appropriate version of GRPC,
  which itself forces updates to protobuf and other dependencies. We
  take the opportunity to remove the protobuf subtree in favor of
  a more sanitized import in the WORKSPACE.
* The various upgrades also upgraded our version of buildifier,
  which now performs more aggressive linting.
* Our version of abseil was upgraded, which now forces you to only
  have access to const references to things stored in absl::btree_set's
  (see https://github.com/abseil/abseil-cpp/commit/a74b796ab3f114f6991479c9ad9e4c1a0dad3a4b).

Change-Id: I870b8f93451056e011cefa3cdf3c5dc01c19a6f9
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
Signed-off-by: Adam Snaider <adsnaider@gmail.com>
diff --git a/aos/events/logging/logfile_utils.h b/aos/events/logging/logfile_utils.h
index fb3fcbf..8a569ba 100644
--- a/aos/events/logging/logfile_utils.h
+++ b/aos/events/logging/logfile_utils.h
@@ -579,9 +579,8 @@
   // The time this data is sorted until.
   monotonic_clock::time_point sorted_until() const { return sorted_until_; }
 
-  // Returns the next sorted message from the log file.  It is safe to call
-  // std::move() on the result to move the data flatbuffer from it.
-  Message *Front();
+  // Returns the next sorted message from the log file.
+  const Message *Front();
   // Pops the front message.  This should only be called after a call to
   // Front().
   void PopFront();
@@ -656,9 +655,8 @@
   // The time this data is sorted until.
   monotonic_clock::time_point sorted_until() const { return sorted_until_; }
 
-  // Returns the next sorted message from the set of log files.  It is safe to
-  // call std::move() on the result to move the data flatbuffer from it.
-  Message *Front();
+  // Returns the next sorted message from the set of log files.
+  const Message *Front();
   // Pops the front message.  This should only be called after a call to
   // Front().
   void PopFront();
@@ -717,9 +715,8 @@
 
   bool started() const;
 
-  // Returns the next sorted message from the set of log files.  It is safe to
-  // call std::move() on the result to move the data flatbuffer from it.
-  Message *Front();
+  // Returns the next sorted message from the set of log files.
+  const Message *Front();
   // Pops the front message.  This should only be called after a call to
   // Front().
   void PopFront();
@@ -787,9 +784,8 @@
     return boot_merger_.started();
   }
 
-  // Returns the next sorted message from the set of log files.  It is safe to
-  // call std::move() on the result to move the data flatbuffer from it.
-  Message *Front();
+  // Returns the next sorted message from the set of log files.
+  const Message *Front();
 
   // Pops the front message.  This should only be called after a call to
   // Front().
@@ -975,7 +971,7 @@
   void QueueUnmatchedUntil(BootTimestamp t);
 
   // Queues m into matched_messages_.
-  void QueueMessage(Message *m);
+  void QueueMessage(const Message *m);
 
   // If a replay_channels_callback was set and the callback returns false, a
   // matched message is popped and true is returned. Otherwise false is