kv_flat_btree_async.cc: fix AioCompletion resource leak

Call AioCompletion::release() if the completion is no longer
needed.

CID 727980 (#1-4 of 4): Resource leak (RESOURCE_LEAK)
  leaked_storage: Variable "aioc" going out of scope leaks
  the storage it points to.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>