This change adds an API that encapsulates information about an app
launched using the library. It also creates a socket-based communication
layer for apps that are launched as child processes; the launching
application listens for connections from launched apps, and once
communication is established, the channel can be used to send updates
to the launching app, or to send commands to the child app.

The change also includes hooks for local, standalone/client and yarn
masters.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #7052 from vanzin/SPARK-8673.