Make node bindings optionally return pre-rendered JSON buffers

[danpat]

If you're using the NodeJS bindings with OSRM, and you're generating large responses, there can be significant overhead simply translating the OSRM JSON structure into a NodeJS object tree.

There are several use-cases where the next step you take is to simply render the NodeJS object into a JSON string, and send it over the network - using NodeJS as your OSRM HTTP server is one example of this.

This PR adds a new optional parameter to all the NodeJS API methods that allows you to configure the format of the response that OSRM returns. It defaults to the current object tree, but if you supply {format: 'json_buffer'}, it will instead cause OSRM to pre-render the JSON tree on a worker thread, then return a NodeJS Buffer object containing the JSON response string.

In cases where you only wanted to return the result to a client over the network anyway, pre-rendering the JSON has a significant speedup. It significantly unburdens the main NodeJS thread, allowing more concurrent work.

Timeline of a 5000x5000 table request and serialization over the network before this change:

Timeline of a 5000x5000 table request and serialization over the network after this change:

The Main Thread here represents the primary NodeJS event loop - less work done here means the event loop stays clear to handle i/O.

The time spent in the pthread_body has increased slightly in the second image - this shows the additional time spent rendering the JSON outside the main thread.

Tasklist

• CHANGELOG.md entry (How to write a changelog entry)
• update relevant Wiki pages
• add tests (see testing documentation
• review
• adjust for comments
• cherry pick to release branch

Requirements / Relations

Link any requirements here. Other pull requests this PR is based on?