pgsql: Set a default autovacuum vacuum_cost_delay value of 20ms, to

Log Message:
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Set a default autovacuum vacuum_cost_delay value of 20ms, to avoid excessive
I/O utilization, per discussion.

While at it, lower the autovacuum vacuum and analyze threshold values to 50
tuples. It is a bit higher (i.e. more conservative) than what I originally
proposed but much better than the old values for small tables.

Modified Files:
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pgsql/doc/src/sgml:
config.sgml (r1.131 -> r1.132)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.131&r2=1.132)
pgsql/src/backend/utils/misc:
guc.c (r1.405 -> r1.406)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.405&r2=1.406)
postgresql.conf.sample (r1.219 -> r1.220)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.219&r2=1.220)