BUG/API: .merge() and .join() on category dtype columns will now preserve category dtype
closes #10409

```
    before     after       ratio
  [6d2293f7] [c352573f]
-  410.11ms   260.46ms      0.64  join_merge.MergeCategoricals.time_merge_cat

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```