PackExtBlockCacheTable: spread extensions over multiple dfs tables

PackExtBlockCacheTable: spread extensions over multiple dfs tables

The existing DfsBlockCache uses a single table for all
extensions (idx, ridx, ...).

This change introduces an implementation of the table
interface that can keep extensions in different cache
tables.
This selects the appropriate cache to use for a specific
PackExt or DfsStreamKey's PackExt type, allowing the
separation of entries from different pack types to help
limit churn in cache caused by entries of differing sizes.

This is especially useful in fine-tuning caches and
influencing interactions by extension type.
For example, a table holding INDEX types only will
not influence evictions of other PackExt types and
vice versa.

The PackExtBlockCacheTable allowing setting the
underlying DfsBlockCacheTables and mappinh directly,
letting users implement and use custom DfsBlockCacheTables.

Change-Id: Icee7b644ef6b600aa473d35645469d6aa1bce345