Make sure rivers get started

With #3782 we changed the execution order of dynamic mapping updates and index operations. We now first send the mapping update to the master node, and then we index the document. This makes sense but caused issues with rivers as they are started due to the cluster changed event that is triggered on the master node right after the mapping update has been applied, but in order for the river to be started its _meta document needs to be available, which is not the case anymore as the index operation most likely hasn't happened yet. As a result in most of the cases rivers don't get started.
What we want to do is retry a few times if the _meta document wasn't found, so that the river gets started anyway.

Closes #4089, #3840