pluggable compression support

Hi,

While hacking on the indirect toast support I felt the need to find out
how make compression formats pluggable to make sure .

In
http://archives.postgresql.org/message-id/20130605150144.GD28067%40alap2.anarazel.de
I submitted an initial patch that showed some promising results.

Here's a more cleaned up version which isn't intermingled with indirect
toast tuple support anymore.

It still contains a guc as described in the above message to control the
algorithm used for compressing new tuples but I think we should remove
that guc after testing.

Greetings,

Andres Freund

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