[WIP] Move ClientContext into StreamApp

[manuzhang]

R: @huafengw

This is work in process and even doesn't compile (breaking RemoteMaterializerImpl). The motivation is to make user application even simpler from

val context = ClientContext(akkaConfig)
val app = StreamApp(name, context)
...
val runningApplication = context.submit(app)
context.close()

to

val app = StreamApp(name, akkaConfig) // creates ClientContext 
val runningApplication: RunningApplication = app.run() // invokes context.submit(app)

and address the following issues I find

1. One subtlety in the current way is StreamApp is implicitly converted to StreamApplication for context.submit. It can be broken if a user forgets to import the conversion (and user doesn't know where it is).
2. I find it difficult to explain the usage of class StreamApp(name: String, system: ActorSystem, userConfig: UserConfig, private val graph: Graph[Op, OpEdge]). We'd better make the constructor private if it's not intended for users.
3. Both RunningApplication and ClientContext have a askAppMaster method to query application status. How should their roles be divided ?
4. RunningApplication has no ScalaDoc.

One downside about the new way is we can't close ClientContext and its underlying ActorSystem. Is it a big problem ?