[kotlin-client] OkHttp call is now non-blocking

Created by: sheepdreamofandroids

All functions in the kotlin-client are marked suspend but the final call to OkHttp was blocking the thread. This is no longer the case.

Note that this can have impact on integration tests that verify backend calls because now it can happen that one call fails and therefore cancels the whole hierarchy of coroutines which possible cancels other backend calls before they even started. That IS desired behaviour though.

This code is a subset of a few patches that are running in production.

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