Improve HTTP test scaling

[MartinKolarik]

Based on benchmarks the HTTP test performs really well in general, but doesn't scale much beyond one/two cores because it's implemented entirely in node, which runs as a single process. We should likely use a pool of workers on each probe.

[patrykcieszkowski]

I/O requests are less efficient when isolated by worker threads, than when they're left to node's asynchronous nature. Besides, every measurement type, with the exception of HTTP runs in isolated process, and manages its resources on its own.