[Firefox] Lazy load network.js in PdfStreamConverter.js

*Inspired by this thread, started by @nnethercote on dev.platform: https://groups.google.com/forum/#!topic/mozilla.dev.platform/sAuhWQOAWic.*

This PR changes `network.js` to be lazily loaded instead, to delay loading it until it's needed and to avoid affecting the browser start-up.

I my testing this works fine, and given that we already lazy load other modules (e.g. `PdfJsTelemetry.jsm`) I don't think that there should be any issues with this change.