Scala 2.10 migration

This PR migrates spark to scala 2.10.

Summary of changes apart from scala 2.10 migration:
(has no implications for user.)
1. Migrated Akka to 2.2.3.

2. Does not use remote death watch for it has a bug, where it tries to send message to dead node infinitely.

3. Uses an indestructible actorsystem which tolerates errors *only* on executors.

(Might be useful for user.)
4. New configuration settings introduced: 

`System.getProperty("spark.akka.heartbeat.pauses", "600")`
`System.getProperty("spark.akka.failure-detector.threshold", "300.0")`
`System.getProperty("spark.akka.heartbeat.interval", "1000")`

Defaults for these are fairly large to only disable Failure detector that comes with akka. The reason for doing so is we have our own failure detector like mechanism in place and then this is just an overhead on top of that + it leads to a lot of false positives. But with these properties it is possible to enable them. A good use case for enabling it could be when someone wants spark to be sensitive (in a controllable manner ofc.) to GC pauses/Network lags and quickly evict executors that experienced it. More information is included in `configuration.md`

Once we have the SPARK-544 merged, I had like to deprecate atleast these akka properties and may be others too.