Fix flaky use-after-free in udp_server

Upgrade third_party/protobuf to 3.2.0 and update C# nugets.

Since the build script of python grpcio-tools package
is very much entangled with the build system of protobuf,
and protobuf build scripts added a build step that
compiles js/embed.cc to create a preprocessor binary
that takes some .js files and generates a single
well_known_types_embed.cc which then gets linked to the
rest of protobuf source tree.

The generated code seems to be used only by the JS
portion of protoc, so is not expected to affect
our python build.  Since we did not want to replicate
the entire build process of protobuf, we decided to
pre-generate the well_known_types_embed.cc and keep
it in our repo and then substitute the js/embed.cc
compiler file with it.

Expose AuthContext in C#

Use gpr_subprocess in fling_stream_test

Upmerge of 1.1.x to master.

Use grpc_build_system.bzl in other projects

Update PROTOCOL-WEB.md

replaced protobuf tag

Add a slice type that shares a refcount with a transport stream

Disable node_protobuf_async_unary_qps_unconstrained benchmark test

Eliminate cancellation if we never sent an op down

Avoid using oversized frames

Lint progress.

Add fixtures for call creation benchmarks

Add licenses() in BUILD files

Remove text-base64 C-T. Will revisit if we ever need support non-default (base64) encoding.

Added //external to grpc deps in case where grpc_build_system.bzl is
imported in a project that is using grpc as a dependency.

Lint fixes.