Merge r4980: Emit Length for client side right before ReqEnd, to summarize ESI transactions correctly.

Only emit Length and ReqEnd if we have an XID.

Fixes: #709
Fixes: #720



git-svn-id: http://www.varnish-cache.org/svn/branches/2.1@5028 d4fa192b-c00b-0410-8231-f00ffab90ce4