Replace heapam.h includes with {table, relation}.h where applicable.

A lot of files only included heapam.h for relation_open, heap_open etc
- replace the heapam.h include in those files with the narrower
header.

Author: Andres Freund
Discussion: https://postgr.es/m/20190111000539.xbv7s6w7ilcvm7dp@alap3.anarazel.de

src/backend/commands/lockcmds.c

@@ -14,6 +14,7 @@
  */
 #include "postgres.h"
 
+#include "access/table.h"
 #include "access/xact.h"
 #include "catalog/namespace.h"
 #include "catalog/pg_inherits.h"
@@ -25,7 +26,6 @@
 #include "utils/lsyscache.h"
 #include "utils/syscache.h"
 #include "rewrite/rewriteHandler.h"
-#include "access/heapam.h"
 #include "nodes/nodeFuncs.h"
 
 static void LockTableRecurse(Oid reloid, LOCKMODE lockmode, bool nowait, Oid userid);