[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/doxyfile

@@ -765,6 +765,7 @@ INPUT = "FlatBuffers.md" \
         "../../CONTRIBUTING.md" \
         "Tutorial.md" \
         "GoApi.md" \
+        "gRPC/CppUsage.md" \
         "groups" \
         "../../java/com/google/flatbuffers" \
         "../../python/flatbuffers/builder.py" \
@@ -883,21 +884,21 @@ EXCLUDE_SYMBOLS        =
 # that contain example code fragments that are included (see the \include
 # command).
 
-EXAMPLE_PATH           = "GoApi_generated.txt"
+EXAMPLE_PATH           = "GoApi_generated.txt" "../../grpc/samples"
 
 # If the value of the EXAMPLE_PATH tag contains directories, you can use the
 # EXAMPLE_PATTERNS tag to specify one or more wildcard pattern (like *.cpp and
 # *.h) to filter out the source-files in the directories. If left blank all
 # files are included.
 
-EXAMPLE_PATTERNS       = *
+EXAMPLE_PATTERNS       = *.cpp *.h *.txt *.fbs
 
 # If the EXAMPLE_RECURSIVE tag is set to YES then subdirectories will be
 # searched for input files to be used with the \include or \dontinclude commands
 # irrespective of the value of the RECURSIVE tag.
 # The default value is: NO.
 
-EXAMPLE_RECURSIVE      = NO
+EXAMPLE_RECURSIVE      = YES
 
 # The IMAGE_PATH tag can be used to specify one or more files or directories
 # that contain images that are to be included in the documentation (see the