Eventhandler to index on command

[cedricve]

It would be helpful to sync ES with S3 at specific events. So for example one could send an action to the river, to start the sync process.

I've seen in your source code, that you're doing a recurring sync process and applying the ListObject function several times, until the truncated flag is set to false. This a costly process for very large buckets. Therefore it would be a nice to have, to only sync ES, when some new files have been added to the S3 bucket, and thus the S3 river could be triggered manually, by an application, instead of recurring.