Vacuum only with 20% old tuples

I suggest that we change vacuum to only move remove tuples if there is
more than 20% expired tuples.

When we do vacuum, we drop all indexes and recreate them.  

This fixes the complaint about vacuum slowness when there are many
expired rows in the table.  We know this is causes by excessive index
updates.  It allows indexes to shrink (Jan pointed this out to me.)  And
it fixes the TOAST problem with TOAST values in indexes.

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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