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: ----------------------------------------------------------------------

Modified Files:
--------------
    pgsql/src/backend/commands:
        vacuumlazy.c (r1.99 -> r1.100)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.99&r2=1.100)