Assert that buffers are marked dirty before XLogRegisterBuffer().

Enforce the rule from transam/README in XLogRegisterBuffer(), and
update callers to follow the rule.

Hash indexes sometimes register clean pages as a part of the locking
protocol, so provide a REGBUF_NO_CHANGE flag to support that use.

Discussion: https://postgr.es/m/c84114f8-c7f1-5b57-f85a-3adc31e1a904@iki.fi
Reviewed-by: Heikki Linnakangas

src/include/access/xloginsert.h

@@ -37,6 +37,7 @@
 									 * will be skipped) */
 #define REGBUF_KEEP_DATA	0x10	/* include data even if a full-page image
 									 * is taken */
+#define REGBUF_NO_CHANGE	0x20	/* intentionally register clean buffer */
 
 /* prototypes for public functions in xloginsert.c: */
 extern void XLogBeginInsert(void);