disk_block_cache: Check shutdownCh under lock
disk_block_cache: Check shutdownCh under lock

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