Improve LLAP CacheContentsTracker to collect and display correct statistics

When keeping track of which buffers correspond to what Hive objects, CacheContentsTracker relies on cache tags.

Currently a tag is a simple String that ideally holds DB and table name, and a partition spec concatenated by . and / . The information here is derived from the Path of the file that is getting cached. Needless to say sometimes this produces a wrong tag especially for external tables.

Also there's a bug when calculating aggregated stats for a 'parent' tag (corresponding to the table of the partition) because the overall maxCount and maxSize do not add up to the sum of those in the partitions. This happens when buffers get removed from the cache.