[native xgb distributed training] allow failed worker retry

[chenqin]

The goal of this pr is to enable native xgb worker retry on both approx and fast hist tree_method,
dmlc/rabit#98

• via enable rabit_cache=1 setting, user can retry failed xgb worker without restart entire cluster
• add rabit_recovery test case to traivs
• add missing parameters to checkpoint payload
• detect older model to be backward compatible

Misc: I will update gitmodules once dependency pr landed.

[trivialfis]

Is it possible to use CMake instead of Make?

[chenqin]

Is it possible to use CMake instead of Make?

Looks like building librabit_mock.a with rabit CMake will not build correctly. xgboost cmake has separate logic to build librabit.a. Looking...

[chenqin]

Is it possible to use CMake instead of Make?
@trivialfis
hum, switched to cmake and passed travis-ci, jenkinsis still failing not sure why

[trivialfis]

@chenqin clang-tidy?

[chenqin]

moved to #4808