Support backpressure for index sync

[philippguertler]

The index sync process currently creates requests for every element in Mesh while sending requests to Elasticsearch. This can cause a lot of buffered documents with high memory usage when elastic search is not fast enough to process the requests.

The generation of the requests already uses Flowables, but this is converted to an observable which does not support backpressure. This should be fixed to only use flowables.

[Jotschi]

Implemented in #791