Avoid caching expression state trees for domain constraints across queries.

In commit 8abb3cda0d I attempted to cache
the expression state trees constructed for domain CHECK constraints for
the life of the backend (assuming the domain's constraints don't get
redefined).  However, this turns out not to work very well, because
execQual.c will run those state trees with ecxt_per_query_memory pointing
to a query-lifespan context, and in some situations we'll end up with
pointers into that context getting stored into the state trees.  This
happens in particular with SQL-language functions, as reported by
Emre Hasegeli, but there are many other cases.

To fix, keep only the expression plan trees for domain CHECK constraints
in the typcache's data structure, and revert to performing ExecInitExpr
(at least) once per query to set up expression state trees in the query's
context.

Eventually it'd be nice to undo this, but that will require some careful
thought about memory management for expression state trees, and it seems
far too late for any such redesign in 9.5.  This way is still much more
efficient than what happened before 8abb3cda0.

src/include/utils/typcache.h

@@ -130,6 +130,7 @@ typedef struct TypeCacheEntry
 typedef struct DomainConstraintRef
 {
 	List	   *constraints;	/* list of DomainConstraintState nodes */
+	MemoryContext refctx;		/* context holding DomainConstraintRef */
 
 	/* Management data --- treat these fields as private to typcache.c */
 	TypeCacheEntry *tcache;		/* owning typcache entry */