Support parallel requests

[avaer]

Exokit hasn't supported (or really needed to support) parallelized dom load so far, but we've been seeing some timeout issues when resources like images and scripts load too slowly (serially) on the initial dom bootstrap.

This issue is to track adding support for parallel "running" of the html on load. This seems pretty simple; I think we just need to keep a refcount of the number of resources being loaded in parallel so we know when to emit DOMContentLoaded.

Note that things other than the initial dom load are already parallelized by node's I/O. This is just about the initial load.

[avaer]

This one has gotten worse for some script-juggling load engines after we implemented properly wait for the manual script attach case in #317.