Refactor configuration [Part II]. (#4577)

* Refactor configuration [Part II].

* General changes:
** Remove `Init` methods to avoid ambiguity.
** Remove `Configure(std::map<>)` to avoid redundant copying and prepare for
   parameter validation. (`std::vector` is returned from `InitAllowUnknown`).
** Add name to tree updaters for easier debugging.

* Learner changes:
** Make `LearnerImpl` the only source of configuration.

    All configurations are stored and carried out by `LearnerImpl::Configure()`.

** Remove booster in C API.

    Originally kept for "compatibility reason", but did not state why.  So here
    we just remove it.

** Add a `metric_names_` field in `LearnerImpl`.
** Remove `LazyInit`.  Configuration will always be lazy.
** Run `Configure` before every iteration.

* Predictor changes:
** Allocate both cpu and gpu predictor.
** Remove cpu_predictor from gpu_predictor.

    `GBTree` is now used to dispatch the predictor.

** Remove some GPU Predictor tests.

* IO

No IO changes.  The binary model format stability is tested by comparing
hashing value of save models between two commits

src/tree/updater_sync.cc

@@ -20,7 +20,11 @@ DMLC_REGISTRY_FILE_TAG(updater_sync);
  */
 class TreeSyncher: public TreeUpdater {
  public:
-  void Init(const std::vector<std::pair<std::string, std::string> >& args) override {}
+  void Configure(const Args& args) override {}
+
+  char const* Name() const override {
+    return "prune";
+  }
 
   void Update(HostDeviceVector<GradientPair> *gpair,
               DMatrix* dmat,