Hosted player: parse the trace in a Web Worker, with progress #497

Part of #494.

Problem

The hosted player unpacks and parses the whole zip synchronously on the page's main thread, so the tab freezes for as long as that takes and gives no progress.

Proposal

Run parseTraceZip in a Web Worker and post the payload back, with a progress indicator while it runs. This removes the freeze; memory use is unchanged (see the on-demand reading sub-issue for that).