dfs: update getBlockCacheStats to return a List of BlockCacheStats

dfs: update getBlockCacheStats to return a List of BlockCacheStats

Make available all underlying cache table stats for the used cache table
implementation.

The existing cache table stats implementation only allows a "global"
view of the cache table statistics; it does not differentiate between
all possible underlying cache tables used.

This change allows callers to get the block cache stats broken down
per underlying table. These cache stats are intended to be used for
monitoring all cache tables independently.

Existing usages of getBlockCacheStats now make use of
AggregatedBlockCacheStats.fromStatsList to aggregate the list of
BlockCacheStats into a single BlockCacheStats instance.

Change-Id: I261b3f2849857172397657e5c674b11e09807f27