BUG/MAJOR: leastconn: Protect tree_elt with the lbprm lock

BUG/MAJOR: leastconn: Protect tree_elt with the lbprm lock

In fwlc_srv_reposition(), set the server's tree_elt while we still hold
the lbprm read lock. While it was protected from concurrent
fwlc_srv_reposition() calls by the server's lb_lock, it was not from
dequeuing/requeuing that could occur if the server gets down/up or its
weight is changed, and that would lead to inconsistencies, and the
watchdog killing the process because it is stuck in an infinite loop in
fwlc_get_next_server().

This hopefully fixes github issue #2990.

This should be backported to 3.2.