cmap: Shrink cmap when load factor is below 20%.

This commit adds check in cmap_remove() and shrinks the cmap by half
if the load factor is below 20%.  This is to reduce the memory
utilization of cmap and to avoid the allocated cmap memory occupying
the top of heap memory, preventing the trim of heap.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>