DfsBlockCache: move cache table specific implementations to a new class

DfsBlockCache: move cache table specific implementations to a new class

This move of the cache table specific implementation to its own class
and extracting the method definition to an interface will allow for
additional reworking of the dfs block cache with the goal of letting
users implement their own context specific cache tables.

This work does not intend to change how the dfs block cache is accessed,
limiting the change to DfsBlockCache internal changes only.

Change-Id: Ief6755d1dcf54b4f73f9fe8d22ccb9e2952cb0c4
Signed-off-by: Laura Hamelin <haowl@google.com>