init: read block cache config options from the config file
init: read block cache config options from the config file

* `kbfs.block_cache.mem_max_bytes`: the max size of the in-memory
  block cache.
* `kbfs.block_cache.disk_max_fraction`: the fraction of the disk's
  free space that the transient disk block cache may use.
* `kbfs.block_cache.sync_max_fraction`: the fraction of the disk's
  free space that the disk sync cache may use.

Issue: HOTPOT-891

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