Add a rxbytes client command to varnishtest
Add Range support to Varnish in streaming mode.

This patch is largely based on work by Mark Bergsma of WikiMedia.
Range patch introduced a situation where threads could get stuck
waiting for end of stream signal indefinetely due to lost signalling.

Since the token strategy was too complex to get right, rewrite it to
something that should be much easier to maintain.

Also clean up the range support a bit, moving all the limit checking
into RES_StreamWrite().