Move materialized views' is-populated status into their pg_class entries.

Previously this state was represented by whether the view's disk file had
zero or nonzero size, which is problematic for numerous reasons, since it's
breaking a fundamental assumption about heap storage.  This was done to
allow unlogged matviews to revert to unpopulated status after a crash
despite our lack of any ability to update catalog entries post-crash.
However, this poses enough risk of future problems that it seems better to
not support unlogged matviews until we can find another way.  Accordingly,
revert that choice as well as a number of existing kluges forced by it
in favor of creating a pg_class.relispopulated flag column.

src/include/commands/matview.h

@@ -20,7 +20,7 @@
 #include "utils/relcache.h"
 
 
-extern void SetMatViewToPopulated(Relation relation);
+extern void SetMatViewPopulatedState(Relation relation, bool newstate);
 
 extern void ExecRefreshMatView(RefreshMatViewStmt *stmt, const char *queryString,
 				  ParamListInfo params, char *completionTag);