Add remote interface for SparkContext

A quick description of the issue: as part of running Hive jobs on top of Spark, it's desirable to have a SparkContext that is running in the background and listening for job requests for a particular user session.

Running multiple contexts in the same JVM is not a very good solution. Not only SparkContext currently has issues sharing the same JVM among multiple instances, but that turns the JVM running the contexts into a huge bottleneck in the system.

So I'm proposing a solution where we have a SparkContext that is running in a separate process, and listening for requests from the client application via some RPC interface (most probably Akka).

I'll attach a document shortly with the current proposal. Let's use this bug to discuss the proposal and any other suggestions.