Fix an inconsequential oversight in session management, and prevent it
from happening again:

We optimize session allocation, SES_New(), to minimize the amount of
locking the VCA_thread participates in with flip-flop lists of free
sessions: Allocate from one list, free to the other, flip lists only
when necessary.

This scheme only works if nobody but VCA_thread calls SES_New().

Background worker threads need dummy sessions and thus called into
SES_New() as well.

These calls all happen during startup, so they do in fact not mess up
the locking, but made mockery of a number of very stern comments.

Add SES_Alloc() for such "other uses" and enforce VCA_thread monopoly
with an assert.



git-svn-id: http://www.varnish-cache.org/svn/branches/2.0@4259 d4fa192b-c00b-0410-8231-f00ffab90ce4