Sync: SDK loops to get All entries without backpressure

[t-botz]

Context

Trying to load Contentful every month to Elastic search, incremental updates done via webhooks.
We have a very large space with 40k+ entries.

Issue

When doing:

 contentfulClient.sync().fetch()

We get

You have exceeded the rate limit of this Space by making too many API requests within a short timespan. Please wait a moment before trying the request again.

Expectation

The SDK should let us have some back pressure. We didn't expected the SDK to loop using the sync rest api and trying to load all entries in memory. Given the SDK use RxJava and Flowable it would make sense to yield each result from the sync API.

We couldn't find an easy to extend the Contentful SDK to do that.