Only move objects to the tail of the LRU queue if we can get the

Only move objects to the tail of the LRU queue if we can get the
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: svn+ssh://projects.linpro.no/svn/varnish/trunk@2534 d4fa192b-c00b-0410-8231-f00ffab90ce4