Improve output packet interleaving

The 'wait' constant was a terrible means of trying to ensure that the
packets were interleaved.  Instead, calculate the current highest
timestamps of each encoder that's present in the interleaved buffer, and
use that as a means of detecting whether the current packet should be
sent off.  This will guarantee sorting without relying on some arbirary
constant that 'assumes' that it'll be interleaved.  It also reduces
buffering any more than what is needed to interleave.