MT-SAFE iteration over LRU L1 (lru_thread).

The unique instance of itertion over any qlane's L1 queue
can be made MT-safe by ensuring that invalidating operations
(only delete, due to queue semantics) also update the
the iteration.

Mark qlanes as active when iteration is in progress, and
update lane iterators iif active.

Fix an apparent reference accounting problem--lru refcnt was
not elevated over lru_thread migration of entries to L2.

Remove redundant while loop checking qlength and per-lane work,
as these are checked in the iteration.

Reorder operations in L1->L2.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>