Preload of precached WebUI triggers throttling at preload nodes

[lidel]

This is similar to #755, but throttling occurs at remote node instead of browser itself.

Problem

Adding 22MB file (#782) to embedded js-ipfs triggers a lot of preload calls.
Those calls are limited to 4-at-one-time, but they finish instantly and after a few dozens*.preload.ipfs.io starts responding with 429 Too Many Requests error:

Solution

Needs to be addressed in js-ipfs (will link PR later)