pgsql: Add a GUC variable "synchronize_seqscans" to allow clients to

Log Message:
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Add a GUC variable "synchronize_seqscans" to allow clients to disable the new
synchronized-scanning behavior, and make pg_dump disable sync scans so that
it will reliably preserve row ordering. Per recent discussions.

Modified Files:
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pgsql/doc/src/sgml:
config.sgml (r1.162 -> r1.163)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.162&r2=1.163)
pgsql/src/backend/access/heap:
heapam.c (r1.248 -> r1.249)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c?r1=1.248&r2=1.249)
pgsql/src/backend/utils/misc:
guc.c (r1.431 -> r1.432)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.431&r2=1.432)
postgresql.conf.sample (r1.235 -> r1.236)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.235&r2=1.236)
pgsql/src/bin/pg_dump:
pg_dump.c (r1.481 -> r1.482)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c?r1=1.481&r2=1.482)