Avoid operations linear in the number of links.

Just in case some bozo makes a chained stream with 272,389 links
 with 16 samples in each (coded at 16 Mbps, including overheads).
This avoids quadratic behavior for simple straight-through
 playback: we no longer do a linear search on each chain boundary
 or each call to op_pcm_tell().
N seeks with M chains still requires O(N*log(M)) work.