Refer to streams internally mostly by an index, not the stream_id.

Inputs no longer know about stream_id, only the stream_index.
This is faster (we used maybe 5% of our time in find_stream),
and will make it easier to add non-HTTP outputs in the future.

Within (HTTP) clients, stream_id also no longer exists, but is replaced
by "url", which really works exactly the same way. url is mostly used during
re-exec, however, so it's less important than it used to be.