disk_block_cache_remote: add an LRU prefetch status cache
disk_block_cache_remote: add an LRU prefetch status cache

To avoid making RPCs to kbfsfuse just to get the status.  Most of the
time, this helper process will be the only one dealing with the git
blocks, so it's ok to assume that the status on the disk cache won't
change much except through this remote.

Issue: KBFS-3708

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client