Generate json in the threadpool

[springmeyer]

This is an experimental PR that moves the JSON response creation to happen, asynchronously, in the C++ threadpool. The motivation here is:

• This will result in less main event loop blocking, since the v8 object creation it replaces was happening synchronously
• This will result in less allocations and therefore less garbage collection churn and pauses, since we're now creating a single JS object to hold the response string rather than many small objects to form up the JSON tree
• Now that we pass across a single string as a response, in the future if we wanted to compress the response it would be easy to do in the threadpool using gzip-hpp before sending across the threadpool.

Caveats:

• This works, but likely needs more testing
• This is probably not as fast as it could be since:
  • std::to_string is used heavily and is know to be not as fast as specialized conversion methods
  • string concatenation is used and requires small C++ allocations. We could likely save on allocations by using a StrCat approach like https://abseil.io/tips/3

/cc @mapbox/maps-api @mapsam