Don't create index when initializing the river

It does not really make sense to create the index when the river starts. Two options:

* The index has been created before indexation starts and it makes sense to me as users probably want to provide a mapping and settings
* The index has not been created. In that case, the first bulk item which will be processed will create the index.

This PR skips index creation when the river starts.

Closes #78.