pg_buffercache report

proposal (quick and dirty draft):

```sql
with a as (
SELECT reldatabase, c.relname, count(*) AS buffers
FROM pg_buffercache b INNER JOIN pg_class c
ON b.relfilenode = pg_relation_filenode(c.oid)
GROUP BY 1, 2
ORDER BY 3 DESC
)
select *, sum(buffers) over ()
from a
order by buffers desc
limit 20;
```