UV_THREADPOOL_SIZE may not be taking effect

[vcabbage]

While doing load testing of raster tiles I found that by setting the UV_THREADPOOL_SIZE environment variable to the number of threads available on my system I got significantly better scalability. This was somewhat unexpected since server.js already sets it to at least 1.5 times the number of cores.

tileserver-gl/src/server.js

Lines 4 to 5 in 644db6c

	process.env.UV_THREADPOOL_SIZE =
	Math.ceil(Math.max(4, require('os').cpus().length * 1.5));

I haven't found an authoritative reference, but https://gist.github.com/rjoydip/36031b3b6d46a9d638e3a97816d2a137 suggests that "You cannot change the size of the thread pool once it is created or entered in the event-loop/worker-thread."

I moved the block of code referenced above to the top of main.js and saw a similar performance increase as I did when setting UV_THREADPOOL_SIZE externally.

Would it be worth submitting a PR for this change?

As a side note, it might be nice to check if UV_THREADPOOL_SIZE is already set so that the user may set it themselves if desired.