Add the number of pinning backends to pg_buffercache's output

Hi,

The last week I twice had the need to see how many backends had some
buffers pinned. Once during development and once while analyzing a stuck
vacuum (waiting for a cleanup lock).
I'd like to add a column to pg_buffercache exposing that. The name I've
come up with is 'pinning_backends' to reflect the fact that it's not the
actual pincount due to the backend private arrays.

I'll add this patch to to 2014-06 CF.

Greetings,

Andres Freund

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 Andres Freund                       http://www.2ndQuadrant.com/
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