Reorganize the code a little, and add code to wait for all threads to finish

Reorganize the code a little, and add code to wait for all threads to finish
processing pending messages before we exit.

Note that VSL_Dispatch() will read in log data as fast as it can, so when
working from a log file, varnishreplay will usually read in the entire file
into memory within the first few seconds.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1623 d4fa192b-c00b-0410-8231-f00ffab90ce4