pgsql: Reduce the size of memory allocations by lazy vacuum when

Log Message:
-----------
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.
CVS: ----------------------------------------------------------------------
CVS: Enter Log. Lines beginning with `CVS:' are removed automatically
CVS:
CVS: Committing in .
CVS:
CVS: Modified Files:
CVS: commands/vacuumlazy.c
CVS: ----------------------------------------------------------------------

Tags:
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REL8_1_STABLE

Modified Files:
--------------
pgsql/src/backend/commands:
vacuumlazy.c (r1.61.2.5 -> r1.61.2.6)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.61.2.5&r2=1.61.2.6)