This reverts commit 79759fea, reversing
changes made to dc36f4df.

php ```is_a``` function expects fully qualified class name to validate.

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

completion_queue_next() loop in grpc_php_shutdown_completion_queue
function as it is no longer needed. This was most likely an artifact of
a previous version of grpc-core where completion queues did not have a
"one tag in, one tag out" rule

This reverts commit 5e01e2ac.