Multipart & URI query parsing, proper WorkerThreads

• Adds a multipart parser, uWS.getParts(body, contentType) that parses at least 160x faster than parse-multipart given 120kb input.
• Adds req.getQuery(key) for fetching the URI decoded URI query. Executes 400x faster than decodeURIComponent, so make sure you use this whenever you can.
• Fixes graceful shutdown of WorkerThreads - no longer will the process fail on libuv assertions when exiting cleanly from a worker thread.
• Bumped ABI required on macOS - you'll need macOS Mojave or newer from now on.
• Fixed a pedantic CodeQL alert - we are ranked flawless A+ now.
• Updates to latest uWS.