Add qid like ranklib format (#2749)

* add qid for https://github.com/dmlc/xgboost/issues/2748

* change names

* change spaces

* change qid to bst_uint type

* change qid type to size_t

* change qid first to SIZE_MAX

* change qid type from size_t to uint64_t

* update dmlc-core

* fix qids name error

* fix group_ptr_ error

* Style fix

* Add qid handling logic to SparsePage

* New MetaInfo format + backward compatibility fix

Old MetaInfo format (1.0) doesn't contain qid field. We still want to be able
to read from MetaInfo files saved in old format. Also, define a new format
(2.0) that contains the qid field. This way, we can distinguish files that
contain qid and those that do not.

* Update MetaInfo test

* Simply group assignment logic

* Explicitly set qid=nullptr in NativeDataIter

NativeDataIter's callback does not support qid field. Users of NativeDataIter
will need to call setGroup() function separately to set group information.

* Save qids_ in SaveBinary()

* Upgrade dmlc-core submodule

* Add a test for reading qid

* Add contributor

* Check the size of qids_

* Document qid format

src/c_api/c_api.cc

@@ -141,6 +141,8 @@ class NativeDataIter : public dmlc::Parser<uint32_t> {
     block_.offset = dmlc::BeginPtr(offset_);
     block_.label = dmlc::BeginPtr(label_);
     block_.weight = dmlc::BeginPtr(weight_);
+    block_.qid = nullptr;
+    block_.field = nullptr;
     block_.index = dmlc::BeginPtr(index_);
     block_.value = dmlc::BeginPtr(value_);
     bytes_read_ += offset_.size() * sizeof(size_t) +