Track stream start suitability separately for each data block added.

Track stream start suitability separately for each data block added.

When having queued data, keep the separate stream start suitability flag
for each iovec instead of just storing the index of the last block.
This is a no-op in itself, but it is a prerequisite for tracking it in
the backlog as well, which we want to do to be able to have force_prebuffer
give out older data -- that requires tracking not only the last suitable
starting point, but multiple ones backwards in time.

Also add a TODO to update starting point(s) when the header changes.