BUG/MINOR: memory: make the thread-local cache allocator set the debugging link

BUG/MINOR: memory: make the thread-local cache allocator set the debugging link

When building with DEBUG_MEMORY_POOLS, an element returned from the
cache would not have its pool link initialized unless it's allocated
using pool_alloc(). This is problematic for buffer allocators which
use pool_alloc_dirty(), as freeing this object will make the code
think it was allocated from another pool. This patch does two things :
  - make __pool_get_from_cache() set the link
  - remove the extra initialization from pool_alloc() since it's always
    done in either __pool_get_first() or __pool_refill_alloc()

This patch is marked MINOR since it only affects code explicitly built
for debugging. No backport is needed.