[pixels-cache,pixel-common,pixels-daemon] add absolute cache balancer

[bianhq]

Currently, the CacheCoordinator uses locality based balancer, which chooses the most free replica host for each file. This is not enough if the files are not balanced from the ground up or when the files have only one replica.

Absolute balancer will ensure each node is allocated the absolute even number of files to cache.

[bianhq]

Issue resolved.