When building segment timelines, don't use the `mimeType` attribute
of the `SegmentTemplate`'s parent node as a fallback when the parent's
`id` attribute is missing.
This doesn't work if the parent node is a `Period`, as it doesn't have
the `mimeType` attribute, and it also generates timeline keys which
are not unique if two streams of the same `mimeType` would get
muxed (currently not supported).
Instead, build an `ident` tuple on the `Representation` instance
consisting of the ids of the parent `Period` and `AdaptationSet`, and
the `Representation` itself, with its required `id` attribute. This
ensures a unique key for the `timelines` dict.
Then simply pass the `ident` tuple to the `SegmentTemplate.segments()`
generator, similar to the `BaseURL` fixes of  e684913 .

Open

Use the newly added `Representation.ident` for finding the same
`Representation` after a manifest reload of a dynamic DASH stream.
Also use the worker's `period` attribute instead of the hardcoded
zero index, even though only the first period is supported.