Reduce the size of memory allocations by lazy vacuum when processing a small

Reduce the size of memory allocations by lazy vacuum when processing a small
table, by allocating just enough for a hardcoded number of dead tuples per
page.  The current estimate is 200 dead tuples per page.

Per reports from Jeff Amiel, Erik Jones and Marko Kreen, and subsequent
discussion.
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CVS: 	commands/vacuumlazy.c
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