MINOR: chunks: Add function to get a large/regular chunk depending on a buffer

MINOR: chunks: Add function to get a large/regular chunk depending on a buffer

get_best_trash_chunk() function was added to be able to get a large or a
regular chunk depending on a given size but never larget than a given
buffer. It will be usefull in a futur fix, to prevent unexpected large chunk
usage.

alloc_best_trash_chunk() function is similar but instead of returning one of
the static chunks, it allocate it from the corresponding pool, large or
regular.