Add a script to run interop tests against all versions of prod servers.

Allow use of alternative credential types.

Upmerge from 0.13.1

Fix windows leaks

PHP: fix examples ClientStub constructor

Make performance benchmarking code more canonical in structure

Redo "Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll()"

Remove tests for EventInvocation API that is going to be removed

Dns retry support

Conflicts:
	composer.json
	src/php/composer.json

Pre 0.13.1 downmerge

Reachability ObjC fix

Node: propagate read errors back down to core

Support for coreclr

Update PHP composer.json files

Add clang3.4 and 3.6 support to run_tests.py

Remove unnecessary safety-refs

Revert "Revert "Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll()""

Move C-layer call creation into GRPCChannel, so that it always acts on a
non-destroyed channel.

Listen for connectivity in the GRPCCall, and destroy the channel.

Fix race between send message and send initial metadata