pgsql: Fix plancache so that any required replanning is done with the

Log Message:
-----------
Fix plancache so that any required replanning is done with the same
search_path that was active when the plan was first made. To do this,
improve namespace.c to support a stack of "override" search path settings
(we must have a stack since nested replan events are entirely possible).
This facility replaces the "special namespace" hack formerly used by
CREATE SCHEMA, and should be able to support per-function search path
settings as well.

Modified Files:
--------------
pgsql/src/backend/catalog:
namespace.c (r1.92 -> r1.93)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/namespace.c.diff?r1=1.92&r2=1.93)
pgsql/src/backend/commands:
schemacmds.c (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/schemacmds.c.diff?r1=1.44&r2=1.45)
pgsql/src/backend/utils/cache:
plancache.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/plancache.c.diff?r1=1.3&r2=1.4)
pgsql/src/include/catalog:
namespace.h (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/namespace.h.diff?r1=1.44&r2=1.45)
pgsql/src/include/utils:
plancache.h (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/plancache.h.diff?r1=1.3&r2=1.4)
pgsql/src/test/regress/expected:
plancache.out (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plancache.out.diff?r1=1.3&r2=1.4)
pgsql/src/test/regress/sql:
plancache.sql (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plancache.sql.diff?r1=1.3&r2=1.4)