Add a finish_buffer function to aggregator, ala videodecoder.

Make it send the segment in a thread safe way.
Slightly fix the infinite_seek test.

Warning: videomixer hasn't been ported.
Warning: baseclass has to reinit the segment for now to have
the correct format.
Warning: we now consider that if we are flush_seeking, we should
not push the buffer. I believe that's right.
These changes make test_infinite_seek pass *nearly* forever, there
is a race where one seek fails, easy to spot, should be reasonably
easy to fix.