adaptivedemux: refactor chunk downloading flow

adaptivedemux: refactor chunk downloading flow

Add more power to the chunk_received function (renamed to data_received)
and also to the fragment_finish function.

The data_received function must parse/decrypt the data if necessary and
also push it using the new push_buffer function that is exposed now. The
default implementation gets data from the stream adapter (all available)
and pushes it.

The fragment_finish function must also advance the fragment. The default
implementation only advances the fragment.

This allows the subsegment handling in dashdemux to continuously download
the same file from the server instead of stopping at every subsegment
boundary and starting a new request