Results not being streamed for lookup_in_all_replicas

At the moment the Ruby SDK's API is not streaming back results for most streaming operations, including lookup_in_all_replicas, as required by the RFCs, but returns an array instead.

This is currently blocked by the C++ core returning the relevant results in an std::vector. Though maybe as a first step we should consider whether any changes are needed in the Ruby SDK's API and if these are possible.