Avoid memory LRU racing disk LRU.

Disk LRU uses the varnish-cache LRU facility, which works by setting
the OC_F_DYING and gaining one reference, resulting in two references.

One is lost again by the thread initiating the LRU nuke, the other by
the EXP thread. Between the two events, the refcnt is one again, thus
stvfe_mutate could race.

I believe this fixes #23 and #24. If not, please reopen