[C++] Improve Allocator handling (#4312)

* Improve Allocator handling

- Templatize Allocator on vector_downward, and make it own the allocator
instance so it can manage lifetimes.
- Templatize + rename FlatBufferBuilderT accordingly, and add a typedef
to FlatBufferBuilder so old code continues to work.
- Fix some issues with the release deleter
- More details in github issue #4311

* Fix constexpr for older MSVC

* Reimplement allocator improvements via inheritance

Instead of templates, use an abstract base class and some unique_ptrs to
implement the new and improved allocator.

* Fix misplaced newline

* Add missing override keyword

* Add macro for func delete to support older compilers

* Explicitly move BufferDeleter (maybe fixes VS 10?)

* Revert previous attempt at VS10 fix

* Try yet another workaround for MS 10

* Use FLATBUFFERS_NOEXCEPT macro

* Add FLATBUFFERS_OVERRIDE macro

* Fix some issues with MSVC 16 hack

* Remove dep on unique_ptr

* Update DEPRECATED macro with a message

* Sigh, no delegating ctors before c++11

* Fix simple_allocator stub

* Relax FlatBufferBuilder ctor

* Add vector_downward reset + assert in clear

* Rename DetachedBuffer to FlatBuffer + remove unnecessary deprecations

* Add flatbuffers::FlatBufferBuilder::Release()

* Address remaining allocator-improvements comments