OPTIM: lb-leastconn: do not take the server lock on take_conn/drop_conn

OPTIM: lb-leastconn: do not take the server lock on take_conn/drop_conn

The operations are only an insert and a delete into the LB tree, which
doesn't require the server's lock at all as the lbprm lock is already
held. Let's drop it. Just for the sake of cleanness, given that the
served and nbpend values used to be atomically updated, we'll use an
atomic load to read them.

(cherry picked from commit 85b2fb03580f60d20d20ecbda8c9b63804d8e900)
Signed-off-by: Willy Tarreau <w@1wt.eu>