Fix cast exception in ScaliakPbClientPool

ScaliakPbClientPool's `withClient` method expects its client pool to build instances that implement `RawClientWithStreaming`.  However its pool factory, `ScaliakPbClientFactory`, merely produces a `RawClient`.

This patch modifies `ScaliakPbClientFactory` to generate `PbStreamingClients` (which implement `RawClientWithStreaming`) instead of `PbClientAdapters` to make  `withClient` happy.