[C++] Improve flatbuffers + gRPC integration (#4310)

* Rework flatbuffers + gRPC integration

- Introduce `flatbuffers::grpc::Message<T>`, a `grpc_slice`-backed
message buffer that handles refcounting and allows flatbuffers to
transfer ownership to gRPC efficiently. This replaces
`flatbuffers::BufferRef<T>`, which required a copy call and was also
unsafe w.r.t. buffer lifetime.
- Introduce `flatbuffers::grpc::MessageBuilder`, a gRPC-specific builder
that forces a `grpc_slice`-backed allocator and also adds some helpful
`Message<T>`-related methods.
- Update serializers accordingly (now zero-copy between flatbuffers and
gRPC).

* gRPC: verify messages by default, but allow user to override

* gRPC: fix some formatting issues

* Disable verification by default, but add helper method

* Make FlatBufferBuilder fields protected + remove vec accessor

* Use bool add_ref parameter to toggle refcount incr

* Remove unnecessary inline specifiers

* Fix formatting

* Use auto

* Remove empty lines

* Use grpc_slice helper macros

* Simplify reset code

* Disable Message copy ctor and assignment by default

* Remove unused member

* Enable gRPC verification by default

* Use auto

* Bake in message verification (remove template specialization)

* Add RoundUp func

* Consolidate gRPC message copy flag

* Make vector_downward allocations fully lazy

* Test message verification failure code/message

* Add grpctest verification test comments

* Simplify reallocate implementation

* Make initial_size a size_t

* Use ternary op for growth_policy

* Use truthiness rather than dont explicit nullptr check

* Indent preprocessor directives

* Remove grpc message copy/assignment

* Fix a few bugs

* Add gRPC example

* Add basic gRPC docs

* Use doxygen EXAMPLE_PATH + @include

* Reference example fbs in grpc docs

* Move gRPC examples into grpc/samples

* Fix pointer/reference formatting

* Use std::function rather than templated callback func

* Create fresh message builder for each request

* Use Clear() in Reset() impl

* Use FLATBUFFERS_CONSTEXPR

docs/source/gRPC/CppUsage.md

@@ -0,0 +1,29 @@
+Use in C++    {#flatbuffers_grpc_guide_use_cpp}
+==========
+
+## Before you get started
+
+Before diving into the FlatBuffers gRPC usage in C++, you should already be
+familiar with the following:
+
+- FlatBuffers as a serialization format
+- [gRPC](http://www.grpc.io/docs/) usage
+
+## Using the FlatBuffers gRPC C++ library
+
+NOTE: The examples below are also in the `grpc/samples/greeter` directory.
+
+We will illustrate usage with the following schema:
+
+@include grpc/samples/greeter/greeter.fbs
+
+When we run `flatc`, we pass in the `--grpc` option and generage an additional
+`greeter.grpc.fb.h` and `greeter.grpc.fb.cc`.
+
+Example server code looks like this:
+
+@include grpc/samples/greeter/server.cpp
+
+Example client code looks like this:
+
+@include grpc/samples/greeter/client.cpp