Add the ability to disable special automatic entities in impl/memory

Add the ability to disable special automatic entities in impl/memory

R=dnj@chromium.org, estaab@chromium.org, maruel@chromium.org, tandrii@chromium.org, vadimsh@chromium.org
BUG=533021
Committed: https://github.com/luci/gae/commit/30614e988d402124209fc53bc78a8e44d4a704f6

[Vadim Sh., 2015-09-24 22:15:27]

lgtm, but why?.. Please add more explanation to the doc string for why this is
useful.

[iannucci, 2015-09-24 22:26:26]

better?

[Vadim Sh., 2015-09-24 22:30:49]

On 2015/09/24 22:26:26, iannucci wrote:
> better?

yeah, little bit

[iannucci, 2015-09-24 23:13:56]

The CQ bit was checked by iannucci@chromium.org

[iannucci, 2015-09-24 23:13:56]

The patchset sent to the CQ was uploaded after l-g-t-m from vadimsh@chromium.org
Link to the patchset: https://codereview.chromium.org/1372473002/#ps20001
(title: "add more docstring")

[commit-bot: I haz the power, 2015-09-24 23:14:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1372473002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1372473002/20001

[commit-bot: I haz the power, 2015-09-24 23:15:29]

Committed patchset #2 (id:20001) as
https://github.com/luci/gae/commit/30614e988d402124209fc53bc78a8e44d4a704f6