Recreated the old PR (https://github.com/grpc/grpc/pull/6283) in a new
repo since the old repo was destroyed. Removed changes to tcp_posix.c
and tpc_windows.c, instead depending on the idempotent endpoint shutdown code
from https://github.com/grpc/grpc/pull/6911.

Load default roots.pem in PHP via grpc_set_ssl_roots_override_callback

Delete tests of dead pre-GA code

Fix secure channel construction

Added third_party/protobuf/src to protoc's include path

Adjust cpu_cost of some tests to better reflect reality

Reduce cost of some heavyweight tests

Unifying error handling within core

Make C++ compile and run under gcc-4.4

Idempotent endpoint shutdown

PHP: implement custom_metadata interop, status_code_and_message, unimplemented_method tests

Update third_party/protobuf to v3.0.0-beta-3.2

Rename FATAL_FAILURE SHUTDOWN

PHP stress client in stress test framework

Disable ccache for ubuntu-14.04/clang3.4 docker config

Allow forcing the use of the default polling strategy [instead of all strategies]