Add support for REFRESH MATERIALIZED VIEW CONCURRENTLY.

This allows reads to continue without any blocking while a REFRESH
runs.  The new data appears atomically as part of transaction
commit.

Review questioned the Assert that a matview was not a system
relation.  This will be addressed separately.

Reviewed by Hitoshi Harada, Robert Haas, Andres Freund.
Merged after review with security patch f3ab5d4.

src/include/nodes/parsenodes.h

@@ -2478,6 +2478,7 @@ typedef struct CreateTableAsStmt
 typedef struct RefreshMatViewStmt
 {
 	NodeTag		type;
+	bool		concurrent;		/* allow concurrent access? */
 	bool		skipData;		/* true for WITH NO DATA */
 	RangeVar   *relation;		/* relation to insert into */
 } RefreshMatViewStmt;