MEDIUM: chunk: Add support for small chunks

MEDIUM: chunk: Add support for small chunks

In the same way support for large chunks was added to properly work with
large buffers, we are now adding supports for small chunks because it is
possible to process small buffers.

So a dedicated memory pool is added to allocate small
chunks. alloc_small_trash_chunk() must be used to allocate a small
chunk. alloc_trash_chunk_sz() and free_trash_chunk() were uppdated to
support small chunks.

In addition, small trash buffers are also created, using the same mechanism
than for regular trash buffers. So three thread-local trash buffers are
created. get_small_trash_chunk() must be used to get a small trash buffer.
And get_trash_chunk_sz() was updated to also deal with small buffers.