[SPARK-15994][MESOS] Allow enabling Mesos fetch cache in coarse executor backend

Mesos 0.23.0 introduces a Fetch Cache feature http://mesos.apache.org/documentation/latest/fetcher/ which allows caching of resources specified in command URIs.

This patch:
- Updates the Mesos shaded protobuf dependency to 0.23.0
- Allows setting `spark.mesos.fetcherCache.enable` to enable the fetch cache for all specified URIs. (URIs must be specified for the setting to have any affect)
- Updates documentation for Mesos configuration with the new setting.

This patch does NOT:
- Allow for per-URI caching configuration. The cache setting is global to ALL URIs for the command.

Author: Charles Allen <charles@allen-net.com>

Closes #13713 from drcrallen/SPARK15994.