Improve categorical concat speed by ~20x
closes #10587

before (current master):

``` python
In [1]: s = pd.Series(list('aabbcd')*1000000).astype('category')

In [2]: timeit pd.concat([s,s])
1 loops, best of 3: 573 ms per loop
```

after (this PR):

``` python
In [1]: s = pd.Series(list('aabbcd')*1000000).astype('category')

In [2]: timeit pd.concat([s,s])
10 loops, best of 3: 30.1 ms per loop
```
