This change allows to execute request filter on every request attempt.
It can be useful i.e. to update credentials when first request fails.
New field `attempt` has been added to `shaka.extern.Request` which
informs which attempt is currently ongoing. This is needed to not do the
unnecessary work multiple times.
HTTP 401 Unauthorized and 403 Forbidden are no longer treated as
immediate critical errors, as tokens can be updated during retry
process.
Wojciech Tyczyński
6d04f7f225