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/backend/commands/tablecmds.c

@@ -3541,7 +3541,8 @@ ATRewriteTables(List **wqueue, LOCKMODE lockmode)
 			heap_close(OldHeap, NoLock);
 
 			/* Create transient table that will receive the modified data */
-			OIDNewHeap = make_new_heap(tab->relid, NewTableSpace);
+			OIDNewHeap = make_new_heap(tab->relid, NewTableSpace, false,
+									   AccessExclusiveLock);
 
 			/*
 			 * Copy the heap data into the new table with the desired