Port single-page btree vacuum logic to hash indexes.

This is advantageous for hash indexes for the same reasons it's good
for btrees: it accelerates space recycling, reducing bloat.

Ashutosh Sharma, reviewed by Amit Kapila and by me.  A bit of
additional hacking by me.

Discussion: http://postgr.es/m/CAE9k0PkRSyzx8dOnokEpUi2A-RFZK72WN0h9DEMv_ut9q6bPRw@mail.gmail.com

src/include/access/hash_xlog.h

@@ -44,6 +44,7 @@
 #define XLOG_HASH_UPDATE_META_PAGE	0xB0		/* update meta page after
 												 * vacuum */
 
+#define XLOG_HASH_VACUUM_ONE_PAGE	0xC0	/* remove dead tuples from index page */
 
 /*
  * xl_hash_split_allocate_page flag values, 8 bits are available.
@@ -250,6 +251,24 @@ typedef struct xl_hash_init_bitmap_page
 #define SizeOfHashInitBitmapPage	\
 	(offsetof(xl_hash_init_bitmap_page, bmsize) + sizeof(uint16))
 
+/*
+ * This is what we need for index tuple deletion and to
+ * update the meta page.
+ *
+ * This data record is used for XLOG_HASH_VACUUM_ONE_PAGE
+ *
+ * Backup Blk 0: bucket page
+ * Backup Blk 1: meta page
+ */
+typedef struct xl_hash_vacuum_one_page
+{
+	RelFileNode	hnode;
+	double		ntuples;
+}	xl_hash_vacuum_one_page;
+
+#define SizeOfHashVacuumOnePage	\
+	(offsetof(xl_hash_vacuum_one_page, ntuples) + sizeof(double))
+
 extern void hash_redo(XLogReaderState *record);
 extern void hash_desc(StringInfo buf, XLogReaderState *record);
 extern const char *hash_identify(uint8 info);