kv_flat_btree_async.cc: fix resource leak

Call AioCompletion::release() if the completion is no longer
needed to free the resources.

CID 727983 : 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>