[SPARK-1769] Executor loss causes NPE race condition

This PR replaces the Schedulable data structures in Pool.scala with thread-safe ones from java. Note that Scala's `with SynchronizedBuffer` trait is soon to be deprecated in 2.11 because it is "unreliable." We should slowly drift away from `SynchronizedBuffer` in other places too.

Note that this PR introduces an API-breaking change; `sc.getAllPools` now returns an Array rather than an ArrayBuffer. This is because we want this method to return an immutable copy rather than one for which modifications take no effect in the original data structure.