DfsBlockCacheConfig: propagate hotmap configs to pack ext cache configs

DfsBlockCacheConfig: propagate hotmap configs to pack ext cache configs

CacheHotMap is currently only set on the base DfsBlockCacheConfig and is
not propagated down to PackExt specific caches.

Because CacheHotMap is set from a method call rather than from Configs,
this change sets per-PackExt CacheHotMap configs on PackExt cache
configs both when DfsBlockCacheConfig#setCacheHotMap(...) is called, and
when DfsBlockCacheConfig#configure(...) is called after setCacheHotMap.

The outer DfsBlockCacheConfig keeps the full CacheHotMap for the same
reason that the CacheHotMap config is propagated in both setCacheHotMap
and configure: the order of operations setting the configuration from
Configs and calling setCacheHotMap is not guaranteed.

Change-Id: Id9dc32fedca99ecc83c9dc90c24d9616873a202e