wait for callback thread

I've checked out the results of using the Protobuf IMU reader without this fix---the ceres-solver does not converge and the GUI doesn't show IMU measurement vectors being added to the camera frames. However, with the changes the ceres-solver does converge.

Note: I'm still not sure of the accuracy of the results; that depends on the relative position of the camera and the IMU, and I'm not sure of the ground truth for the datasets I'm operating on.