Vacuum Delay feature

The attached patch applies to CVS tip as of 02/05/2004 and implements
the cost based vacuum delay feature.

A detailed description with charts of different configuration settings
can be found here:

http://developer.postgresql.org/~wieck/vacuum_cost/

There is a problem left that seems to be related to Toms observations in
the shutdown behaviour of the postmaster. My current guess is that the
napping done via select(2) somehow prevents responding to the query
abort signal.

Jan

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