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/backend/access/rmgrdesc/hashdesc.c

@@ -154,6 +154,8 @@ hash_identify(uint8 info)
 		case XLOG_HASH_UPDATE_META_PAGE:
 			id = "UPDATE_META_PAGE";
 			break;
+		case XLOG_HASH_VACUUM_ONE_PAGE:
+			id = "VACUUM_ONE_PAGE";
 	}
 
 	return id;