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/logging.cc

@@ -50,7 +50,7 @@ bool ConsoleLogger::ShouldLog(LogVerbosity verbosity) {
   return verbosity <= global_verbosity_ || verbosity == LV::kIgnore;
 }
 
-void ConsoleLogger::Configure(const std::map<std::string, std::string>& args) {
+void ConsoleLogger::Configure(Args const& args) {
   param_.InitAllowUnknown(args);
   // Deprecated, but when trying to display deprecation message some R
   // tests trying to catch stdout will fail.