[MRG] Cache features value for extra trees

Caching feature values speed up extra trees by a fair amount  for only 10 lines of code change.

Here some benchmark results on the covertype dataset
```
Classifier   train-time test-time error-rate
--------------------------------------------
master    52.4233s   0.4902s     0.0381 
this pr    44.4372s   0.4892s     0.0381  
```

Cheers !

cc @glouppe