BUG/MEDIUM: stick-table: use the update lock when reading tables from peers

BUG/MEDIUM: stick-table: use the update lock when reading tables from peers

In 2.9, the stick-tables' locking was split between the lock used to
manipulate the contents (->lock) and the lock used to manipulate the
list of updates and the update indexes (->updt_lock). This was done
with commit 87e072eea5 ("MEDIUM: stick-table: use a distinct lock for
the updates tree"). However a part was overlooked in the peers code,
the parts that consult (and update) the indexes use the table's lock
instead of the update lock. It's surprising that it hasn't caused more
trouble. It's likely due to the fact that the tree nodes are not often
immediately freed and that their memory area remains connected to valid
nodes in the tree during peer_stksess_lookup(), while other parts only
check or update indexes, thus are not that critical.

This needs to be backported wherever the commit above is, thus logically
2.9.