Log Message:
Update to 1.4.21:

Fixes

    makefile cleanups
    Avoid OOM errors when locked items stuck in tail

If clients occasionally fetch many items, more than can fit the
TCP buffers, then hang for a very long period of time, that slab
class could OOM. In older versions this could cause a crash. Since
1.4.20 this will cause OOM errors.

Now, if a locked item lands in the LRU tail, it will be bumped back
to the head and an lrutail_reflocked counter incremented. If you're
concerned about having stuck clients, watch that counter.

Big thanks to Jay Grizzard et all at Box for helping track this
down!