[jvm-packages] Scala implementation of the Rabit tracker. (#1612)

* [jvm-packages] Scala implementation of the Rabit tracker.

A Scala implementation of RabitTracker that is interface-interchangable with the
Java implementation, ported from `tracker.py` in the
[dmlc-core project](https://github.com/dmlc/dmlc-core).

* [jvm-packages] Updated Akka dependency in pom.xml.

* Refactored the RabitTracker directory structure.

* Fixed premature stopping of connection handler.

Added a new finite state "AwaitingPortNumber" to explicitly wait for the
worker to send the port, and close the connection. Stopping the actor
prematurely sends a TCP RST to the worker, causing the worker to crash
on AssertionError.

* Added interface IRabitTracker so that user can switch implementations.

* Default timeout duration changes.

* Dependency for Akka tests.

* Removed the main function of RabitTracker.

* A skeleton for testing Akka-based Rabit tracker.

* waitFor() in RabitTracker no longer throws exceptions.

* Completed unit test for the 'start' command of Rabit tracker.

* Preliminary support for Rabit Allreduce via JNI (no prepare function support yet.)

* Fixed the default timeout duration.

* Use Java container to avoid serialization issues due to intermediate wrappers.

* Added tests for Allreduce/model training using Scala Rabit tracker.

* Added spill-over unit test for the Scala Rabit tracker.

* Fixed a typo.

* Overhaul of RabitTracker interface per code review.

  - Removed methods start() waitFor() (no arguments) from IRabitTracker.
  - The timeout in start(timeout) is now worker connection timeout, as tcp
    socket binding timeout is less intuitive.
  - Dropped time unit from start(...) and waitFor(...) methods; the default
    time unit is millisecond.
  - Moved random port number generation into the RabitTrackerHandler.
  - Moved all Rabit-related classes to package ml.dmlc.xgboost4j.scala.rabit.

* More code refactoring and comments.

* Unified timeout constants. Readable tracker status code.

* Add comments to indicate that allReduce is for tests only. Removed all other variants.

* Removed unused imports.

* Simplified signatures of training methods.

 - Moved TrackerConf into parameter map.
 - Changed GeneralParams so that TrackerConf becomes a standalone parameter.
 - Updated test cases accordingly.

* Changed monitoring strategies.

* Reverted monitoring changes.

* Update test case for Rabit AllReduce.

* Mix in UncaughtExceptionHandler into IRabitTracker to prevent tracker from hanging due to exceptions thrown by workers.

* More comprehensive test cases for exception handling and worker connection timeout.

* Handle executor loss due to unknown cause: the newly spawned executor will attempt to connect to the tracker. Interrupt tracker in such case.

* Per code-review, removed training timeout from TrackerConf. Timeout logic must be implemented explicitly and externally in the driver code.

* Reverted scalastyle-config changes.

* Visibility scope change. Interface tweaks.

* Use match pattern to handle tracker_conf parameter.

* Minor clarification in JNI code.

* Clearer intent in match pattern to suppress warnings.

* Removed Future from constructor. Block in start() and waitFor() instead.

* Revert inadvertent comment changes.

* Removed debugging information.

* Updated test cases that are a bit finicky.

* Added comments on the reasoning behind the unit tests for testing Rabit tracker robustness.

jvm-packages/xgboost4j-example/src/main/scala/ml/dmlc/xgboost4j/scala/example/CustomObjective.scala

@@ -57,7 +57,7 @@ object CustomObjective {
         case e: XGBoostError =>
           logger.error(e)
           null
-        case _ =>
+        case _: Throwable =>
           null
       }
       val grad = new Array[Float](nrow)