This commit introduces a global cache replacement (instead of per-client
cleanup).

If one bufio client uses the cache heavily and another client is not using
it, we want to let the first client use most of the cache. The old
algorithm would partition the cache equally betwen the clients and that is
sub-optimal.

For cache replacement, we use the clock algorithm because it doesn't
require taking any lock when the buffer is accessed.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>