This allows Mesos deployments to use the shuffle service (and implicitly dynamic allocation). It does so by adding a new "main" class and two corresponding scripts in `sbin`:

- `sbin/start-shuffle-service.sh`
- `sbin/stop-shuffle-service.sh`

Specific options can be passed in `SPARK_SHUFFLE_OPTS`.

This is picking up work from #3861 /cc tnachen

Author: Iulian Dragos <jaguarul@gmail.com>

Closes #4990 from dragos/feature/external-shuffle-service and squashes the following commits:

6c2b148 [Iulian Dragos] Import order and wrong name fixup.
07804ad [Iulian Dragos] Moved ExternalShuffleService to the `deploy` package + other minor tweaks.
4dc1f91 [Iulian Dragos] Reviewer’s comments:
8145429 [Iulian Dragos] Add an external shuffle service that can be run as a daemon.