common: refactor of lru

common: refactor of lru

Avoids an unnecessary "max" size of the LRU which was used to calculate the
midpoint. Instead, just dynamically move the LRUObjects between top and bottom
on-the-fly.

This change is necessary for a cache which which does not limit by the number
of objects but by some other metric. (In this case, memory.)

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>