rtmp: Support reading interleaved chunks.

A given packet won't always come in contiguously; sometimes
they may be broken up on chunk boundaries by packets of another
channel.

This support primarily involves tracking information about the
data that's been read, so the reader can pick up where it left
off for a given channel.

As a side effect, we no longer over-report the bytes read if
(toread = MIN(size, chunk_size)) == size

Signed-off-by: Martin Storsjö <martin@martin.st>