No possibility to read 'old' XGBoost binary files after #4763

[ShvetsKS]

In PR #4763 the number of fields was extended(change line: https://github.com/dmlc/xgboost/pull/4763/files#diff-95d8126e28130173959c24f455e01a05R42)

It leads to a problem when user is reading saved XGBoost binary file by version of library before #4763 PR.
CHECK_EQ(field_cnt, kNumField) << "Wrong number of fields";

Is it expected behavior? Should we introduce possibility to read 'old' XGBoost binary files?

example:
dtrain = xgb.DMatrix('dtrain_2.bin') # 'dtrain_2.bin' was saved on commit #5437, and no possibility to read this file on #4763

[trivialfis]

It's was decided 1.0 should make this change for future extensibility, and we don't want to maintain any redundant code. So it's a breaking change.

[ShvetsKS]

@trivialfis Thanks for fast response!