Support pg_buffercache

[Krysztophe]

It could provide:

• number of used blocks and/or a % of shared_buffers
  (will max to shared_buffers and 100%, except if filtered on a DB)
• number and % of dirty blocks

maybe split by db

Perhaps is it possible to target a specified table, db,or set of tables.

If filtering by DB or table, add a column showing with percentage of the table are in the shared_buffers.

Usage counts and pins are probably overkill.

[gleu]

Filtering is not done yet. I still don't know how to specify a target kind (table, database). I'll have to think about that.

[Krysztophe]

Un peu austère et ça manque d'unités lisibles (pour #25), mais 👍

Et cette rapidité motive pour d'autres issues :-)