sqlite3 OPFS Worker concurrency tester

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OPFS concurrency tester using multiple independent Workers. Disclaimer: concurrency in OPFS is currently a pain point!

URL flags: pass a number of workers using the `workers=N` URL flag. Set the time between each workload with `interval=N` (milliseconds). Set the number of worker iterations with `iterations=N`. Enable OPFS VFS verbosity with `verbose=1-3` (output goes to the dev console). Disable/enable "unlock ASAP" mode (higher concurrency, lower speed) with `unlock-asap=0-1`.

Achtung: if it does not start to do anything within a couple of seconds, check the dev console: Chrome sometimes fails to load the wasm module due to "cannot allocate WasmMemory." Closing and re-opening the tab usually resolves it, but sometimes restarting the browser is required.

Reverse log order?