Use System.Text.Json in HAL Server and Http Client #285

We'd like to take advantage of the lower allocation, higher performance JSON bits when dotnet core 3.0 is released. This will likely need a change to the Halcyon library we consume - perhaps another method to write the json response directly to a stream via Utf8JsonWriter. We will also need a way to validate incoming json via AppendStreamOperation and SetStreamMetadataOperation without buffering the entire request into memory.