The lock order is lru->exp lock, so that the timer_index
can be safely examined while holding only the lru lock,
eliminating the need for the separate OC_F_ONLRU flag.

The critical trick is that in EXP_Touch() the lru_lock
is sufficient:  We just move the object around on the
lru list, we don't add or delete it.

This hopefully reduces lock contention on these locks.