refactor NodeRequest to stop leaking

[mikemorris]

Stupid fancy refactored code still fails leak test. Will investigate further tomorrow.

/cc @tmpsantos

[tmpsantos]

My understanding is that v8::Function has its lifetime bound to the context.

[mikemorris]

This pull request now implements an alternative to #5527 that solves the v8::FunctionTemplate memory leak while preserving the callback-passing function factory API, but at the cost of creating a new v8::Context for each callback it returns to JavaScript-land.

This is roughly equivalent to creating an iframe to run each function in, and seems to be necessary because of the architecture of v8::FunctionTemplate and v8::Function, which are designed to only be called once in a single context (the global context by default), and calling the function simply creates a new instance of it.

The problem here is that attaching data (like a reference to the NodeRequest object) must be done early, during the creation of the v8::FunctionTemplate, and an instance of the function cannot be created in C++ then passed on to JavaScript (to attach data later with Nan::SetInternalFieldPointer or v8::SetHiddenValue) as calling v8::Function::NewInstance actually executes the function.

In terms of CPU time and memory, it might seem an expensive operation to create a new execution context given the number of built-in objects that must be built. However, V8's extensive caching ensures that, while the first context you create is somewhat expensive, subsequent contexts are much cheaper.

According to the V8 Embedder's Guide, this shouldn't be TOO costly, and has been benchmarked to verify that performance is acceptable.

Changing the API to req.respond, as in #5527, could still possibly be the more performant option, albeit less idiomatic JavaScript, and may be worth considering in the future.

/cc @miccolis @bsudekum @tmpsantos @brunoabinader @jfirebaugh

[jfirebaugh]

This looks good to me.

I wonder what the reason is for the v8 design decision to make FunctionTemplates non-garbage collectable... pretty crazy that you have to make an entire context just to get a temporary function.