Make GRXWriteable with single handler robust against streaming Writers

Fix code coverage.

Add support for IP Addresses in Subject Alt Names.

Fix some race conditions

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