Improve caching.
Improve caching.

 - Minor changes to comments.
 - Remove todo.
 - The assumption that label values do not have -1 is not correct.
   Labels can have any value an int can have, therefore
   an extra variable is added that records when a label
   is already cached.
 - remove duplicate code that was also present in
   cached_label_short.

vbloemen/ltsmin,utwente-fmt/ltsmin,trolando/ltsmin,vbloemen/ltsmin,alaarman/ltsmin,utwente-fmt/ltsmin,alaarman/ltsmin,Meijuh/ltsmin,alaarman/ltsmin,Meijuh/ltsmin,vbloemen/ltsmin,alaarman/ltsmin,Meijuh/ltsmin,alaarman/ltsmin,vbloemen/ltsmin,trolando/ltsmin,Meijuh/ltsmin,vbloemen/ltsmin,Meijuh/ltsmin,utwente-fmt/ltsmin,vbloemen/ltsmin,alaarman/ltsmin,trolando/ltsmin,utwente-fmt/ltsmin,utwente-fmt/ltsmin,utwente-fmt/ltsmin,vbloemen/ltsmin,trolando/ltsmin,Meijuh/ltsmin