Microvacuum for GIST

Mark index tuple as dead if it's pointed by kill_prior_tuple during
ordinary (search) scan and remove it during insert process if there is no
enough space for new tuple to insert. This improves select performance
because index will not return tuple marked as dead and improves insert
performance because it reduces number of page split.

Anastasia Lubennikova <a.lubennikova@postgrespro.ru> with
 minor editorialization by me

src/backend/access/gist/gistscan.c

@@ -93,6 +93,11 @@ gistbeginscan(PG_FUNCTION_ARGS)
 		memset(scan->xs_orderbynulls, true, sizeof(bool) * scan->numberOfOrderBys);
 	}
 
+	so->killedItems = NULL;		/* until needed */
+	so->numKilled = 0;
+	so->curBlkno = InvalidBlockNumber;
+	so->curPageLSN = InvalidXLogRecPtr;
+
 	scan->opaque = so;
 
 	/*