pgsql: Fix domain_in() bug exhibited by Darcy Buskermolen.

Log Message:
-----------
Fix domain_in() bug exhibited by Darcy Buskermolen.  The idea of an EState
that's shorter-lived than the expression state being evaluated in it really
doesn't work :-( --- we end up with fn_extra caches getting deleted while
still in use.  Rather than abandon the notion of caching expression state
across domain_in calls altogether, I chose to make domain_in a bit cozier
with ExprContext.  All we really need for evaluating variable-free
expressions is an ExprContext, not an EState, so I invented the notion of a
"standalone" ExprContext.  domain_in can prevent resource leakages by doing
a ReScanExprContext on this rather than having to free it entirely; so we
can make the ExprContext have the same lifespan (and particularly the same
per_query memory context) as the expression state structs.

Modified Files:
--------------
    pgsql/src/backend/executor:
        execUtils.c (r1.138 -> r1.139)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execUtils.c.diff?r1=1.138&r2=1.139)
    pgsql/src/backend/utils/adt:
        domains.c (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/domains.c.diff?r1=1.2&r2=1.3)
    pgsql/src/include/executor:
        executor.h (r1.127 -> r1.128)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/executor.h.diff?r1=1.127&r2=1.128)
    pgsql/src/include/nodes:
        execnodes.h (r1.157 -> r1.158)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/execnodes.h.diff?r1=1.157&r2=1.158)