expiry mutex without waiting.

This is in addition to the already present "only if it have not
been moved recently" check.

This additional mutex-contestion reduction obviously might leave
the LRU list badly out of order, but this can be worked around
by examining obj.last_use in vcl_discard()



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@2534 d4fa192b-c00b-0410-8231-f00ffab90ce4