Servo currently completely ignores `<meta charset>` tags. When we find
one with an encoding that is incompatible to the current one, then we
should reload the page and start over with the new encoding. A common
optimization that has even made its way into the specification is to
wait for a few bytes to arrive and inspect them for `meta` tags, so the
browser is able to use the correct encoding from the very beginng.
In practice, I've run into problems with our WPT harness when reloading
the page after `meta` tags. Therefore, this change implement the
optimization first, so we never have to reload when running WPT. I've
implemented prescanning in a way where we wait for 1024 bytes to arrive
or for one second to pass, whichever one happens first.
This causes a large number of web platform tests to flip around. I've
looked at most of the new failures and I believe they're reasonable.
Testing: New tests start to pass.
Part of https://github.com/servo/servo/issues/6414
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Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
