Clogged up work queue can delay destruction

[ojura]

If clients of libcartographer wish to abort all operations by destructing their Cartographer instance, this should happen immediately, and not result in blocking because of a filled up work queue in global SLAM. An example is the ROS node blocking on Ctrl-C, instead of exiting immediately.

Referencing closed https://github.com/cartographer-project/cartographer/pull/1446 as an incomplete proposal which needs further work.