Retire sessions if the workspace size changes, properly cache the

Retire sessions if the workspace size changes, properly cache the
workspace size so we do not get caught unaware when it changes.

Implement flip-flop free queue where SES_New() can read from
one of them without a lock, which frees happen to the other one
under lock.  If the lock-less queue is empty, SES_New() flips
the two queues under lock and tries again.  If that queue is
also empty call malloc(3).

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@873 d4fa192b-c00b-0410-8231-f00ffab90ce4