kv_flat_btree_async.cc: fix AioCompletion resource leak

Call AioCompletion::release() if the completion is no longer needed.

CID 727979 (#1-2 of 2): Resource leak (RESOURCE_LEAK)
  leaked_storage: Variable "a" going out of scope leaks the storage
  it points to.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>