Fix race in impl/memory.

Fix race in impl/memory.

R=dnj@chromium.org, estaab@chromium.org, vadimsh@chromium.org
BUG=https://github.com/luci/gae/issues/16

Committed: https://github.com/luci/gae/commit/525a1181d6016089e0eafceb704c840c63b23b2e

[iannucci, 2015-11-06 23:25:03]

https://chromiumcodereview.appspot.com/1411573014/diff/1/impl/memory/datastor...
File impl/memory/datastore_data.go (left):

https://chromiumcodereview.appspot.com/1411573014/diff/1/impl/memory/datastor...
impl/memory/datastore_data.go:223: ents := d.mutableEnts(incomplete.Namespace())
apparently getting a collection locked, releasing the lock, then writing to it
with the lock again is not safe according to the race detector. Now we always
re-acquire the collection when we want to write to it.

[Vadim Sh., 2015-11-06 23:44:20]

lgtm

[iannucci, 2015-11-06 23:44:28]

The CQ bit was checked by iannucci@chromium.org

[commit-bot: I haz the power, 2015-11-06 23:44:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1411573014/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1411573014/1

[commit-bot: I haz the power, 2015-11-06 23:47:07]

Committed patchset #1 (id:1) as
https://github.com/luci/gae/commit/525a1181d6016089e0eafceb704c840c63b23b2e