Requires synchronous and single-threaded, so only 1 channel and 1 outstanding RPC
per channel for now. No open-loop for now

This reverts commit 5e01e2ac.

Add response parameters to custom_metadata streaming request for Node and PHP
clients.

The Java server does not respond with separate initial and trailing
metadata when there is no response data - it is only emiting the
requested trailing metadata. Adding the response parameters to the test
(in accordance with the specification) avoids this, but I will open a
separate issue to investigate the Java behavior.