BUG/MINOR: peers: some updates are pushed twice after a resync.

This bug is due to a copy/paste error and was introduced
with peers-protocol v2:

The last_pushed pointer was not correctly reset to the teaching_origin at
the end of the teaching state but to the first update present in the tree.

The result: some updates were re-pushed after leaving the teaching state.

This fix needs to be backported to 1.6.

src/peers.c

@@ -1491,9 +1491,7 @@ static void peer_io_handler(struct appctx *appctx)
 									/* push local updates */
 									if (!eb || eb->key > st->teaching_origin) {
 										st->flags |= SHTABLE_F_TEACH_STAGE2;
-										eb = eb32_first(&st->table->updates);
-										if (eb)
-											st->last_pushed = eb->key - 1;
+										st->last_pushed = st->teaching_origin;
 										break;
 									}