Rewrite approx (#7214)

This PR rewrites the approx tree method to use codebase from hist for better performance and code sharing.

The rewrite has many benefits:
- Support for both `max_leaves` and `max_depth`.
- Support for `grow_policy`.
- Support for mono constraint.
- Support for feature weights.
- Support for easier bin configuration (`max_bin`).
- Support for categorical data.
- Faster performance for most of the datasets. (many times faster)
- Support for prediction cache.
- Significantly better performance for external memory.
- Unites the code base between approx and hist.

src/gbm/gbtree.cc

@@ -168,7 +168,7 @@ void GBTree::ConfigureUpdaters() {
       // calling this function.
       break;
     case TreeMethod::kApprox:
-      tparam_.updater_seq = "grow_histmaker,prune";
+      tparam_.updater_seq = "grow_histmaker";
       break;
     case TreeMethod::kExact:
       tparam_.updater_seq = "grow_colmaker,prune";