livesync: Wait for the end timestamp of the previous buffer before looking at queue

Previously livesync was waiting for the start timestamp of the current buffer after looking at the queue and right before pushing it downstream. This meant that it generally looked too early at the queue and especially that upstream had to provide the next buffer already at the start timestamp of the previous one.

Instead, now wait before looking at the queue and wait for the end timestamp of the previous buffer. Once the previous buffer has expired, a new buffer really needs to be available or otherwise a filler buffer has to be pushed downstream.