BUGFIX: Add index that should exist but does not

[kdambekalns]

Adds an index that should exist but is not picked up by Doctrine, and
thus was deleted by accident in 20170110130253.

See #1375

[bwaidelich]

@kdambekalns why is this index not picked up by doctrine and does that mean that future migrationgenerate calls will drop it again? And, if so, shouldn't we add another index to https://github.com/neos/neos-development-collection/blob/master/Neos.Neos/Classes/EventLog/Domain/Model/Event.php#L23-L28 ?

[kdambekalns]

The index probably is not picked up because of the single table inheritance involved. It is only contained in https://github.com/neos/neos-development-collection/blob/3.0/Neos.Neos/Classes/EventLog/Domain/Model/NodeEvent.php#L31-L35

So, yes, any new migration will pick it up again, but since the dropping of an index unrelated to whatever one is working on should (tm) be noticed when checking the migrations… well. :)

[kdambekalns]

But… adding it to the Event class might be worth a try. It just looks weird (too), because the property "does not exist" there.