DFS block cache: allow multiple passes for blocks before eviction

DFS block cache: allow multiple passes for blocks before eviction

Let certain pack extensions that are expensive to load from storage
(e.g. pack index, bitmap index) stay in DFS block cache longer than
others by overriding default cache count through DfsBlockCacheConfig

Don't change default behavior when cache override map is empty. Use int
cacheCount instead of boolean hot for Ref<T>

Signed-off-by: Alina Djamankulova <adjama@google.com>
Change-Id: I18062784ec9cc14dbba3e4bb8d9509440cf2d44f