C++-side refactor

[addaleax]

quic: remove typedef enum

Using typedef enum is a C-ism.

quic: remove user_data from quic buffer callback

Using std::function and an opaque pointer are pretty much
antithetical. This commit chooses the more C++-y variant, i.e.
std::function.

quic: cleanup node_quic_buffer.h

Prefer inline functions over macros, use explicit nullptr checks,
do not mark functions with implicit inline linkage as inline,
re-use code where possible, etc.

src: cleanup histogram code

Move methods that do not need to be inline into their own file,
replace std::function with a template variant for performance,
remove inline for functions with implicit inline linkage.

quic: remove opaque pointer from Timer callback

Use the more C++-y std::function approach which renders opaque
pointers unnecessary.

quic: cleanup QuicSession code

• Do not create new v8::Function objects, both for performance
  and because it is unclear whether that leaks memory
  (Node.js issue 28988).
• Do not put functions into the inline header that do not need to
  be there or that do not make sense as inline functions
  (in particular, callbacks that are provided to C libraries can
  not be realized as inline functions by the compiler).
• Remove implicit/redundant inline qualifiers and add appropriate
  const qualifiers.
• Remove unnecessary this-> prefixes.
• Return early for JS failures rather than accumulating them.
• Minor stylistic changes.

src: refactor memory tracker implementation

Use a template class instead of virtual methods to allow inlining
the subclass methods, as well as other minor cleanups.

http2: use shared memory tracking implementation

The extensive testing done on http2 makes it easier to make sure
the implementation is correct (and doesn’t diverge unnecessarily).

quic: remove QuicStreamListener

This listener currently had the drawbacks of leaking memory
when an error is encountered and was otherwise equivalent to
the default listener.

quic: more general C++ cleanup

Use default initializers, const qualifiers, minor style changes
to match more common style in the codebase.

[addaleax]

Landed in 72997d6...ea095af