[DIST] Enable multiple thread make rabit and xgboost threadsafe

[tqchen]

…e thread-safe by using thread local variables.

[tqchen]

cc @CodingCat see newest version of https://github.com/tqchen/xgboost/blob/master/jvm-packages/xgboost4j-demo/src/main/java/ml/dmlc/xgboost4j/demo/DistTrain.java

[tqchen]

• Currently the copy of libxgboost4j library and tracker.py is by create_jni.sh
• We will want to use pom and maven to handle these automatically in the future

[tqchen]

@CodingCat it seems to me that the interface Booster is not necessary. As we will only have one version of Booster each, for scala and java.

[tqchen]

please also help review the serializable commit tqchen@0df2ed8 I feel I did it right.

[CodingCat]

@tqchen I made some changes to the current implementation, mainly for adding some trackable logging statements. and removed the FileUtil, as that static function is only used by NativeLibLoader

for Booster interface , let's leave it there, I'm not sure if we will have more Impl in the projects relying on the core xgboost4j....

PR: tqchen#2