Write GlobalSlamResultCallback RFC. (#31)

text/0020-global-slam-callback.md

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+# Global SLAM Result Callback
+
+## Summary
+[summary]: #summary
+
+For local SLAM a `LocalSlamResultCallback` can be registered when creating new trajectories which is invoked whenever new `RangefinderData` is added to the trajectory.
+Similarly we propose to add a `GlobalSlamResultCallback` which is invoked whenever the optimization problem has been solved.
+
+## Motivation
+[motivation]: #motivation
+
+[Issue #762](https://github.com/googlecartographer/cartographer_ros/issues/762) describes bandwidth issues when publishing the `/trajectory_node_list`.
+To remedy this [Issue #1022](https://github.com/googlecartographer/cartographer/issues/1022) proposes the introduction of a mechanism to signal the completion of an optimization problem.
+The introduction of a `GlobalSlamResultCallback` would accomplish that.
+
+Moreover for cloud-based mapping we would like to the ability to stream the robots pose to the cloud.
+With the introduction of a `GlobalSlamResultCallback` this code would look much more natural as upload would occur as a result of the callback invocation.
+
+## Approach
+[approach]: #approach
+
+We propose to introduce a callback with the following signature:
+
+```
+using GlobalSlamResultCallback = std::function<void(
+    const std::map<int /* trajectory_id */, SubmapId>&,
+    const std::map<int /* trajectory_id */, NodeId>&)>;
+```
+
+i.e. the callback gets passed two maps, mapping trajectory IDs to the last optimized `SubmapId` and `NodeId`.
+
+We would change the public API by adding the `GlobalSlamResultCallback` as a constructor argument to `MapBuilder` which passes it on as a const'or argument to either `PoseGraph2d` or `PoseGraph3d`.
+The callback is invoked in `WhenDone` callback right after `PoseGraph2/3d::RunOptimization()`. We need to take a mutex lock to determine the last optimized `NodeId` and `SubmapId`, but we will give up the lock before invoking the callback to ensure that the provided callback can access the `PoseGraph`.
+
+## Discussion Points
+[discussion]: #discussion
+