MINOR: lb/chash: use a read lock in chash_get_server_hash()

MINOR: lb/chash: use a read lock in chash_get_server_hash()

When using a low hash-balance-factor value, it's possible to loop
many times trying to find the best server. Figures in the order of
100-300 times were observed for 1000 servers with a factor of 101
(which seems a bit excessive for such a large farm). Given that
there's nothing in that function that prevents multiple threads
from working in parallel, let's switch to a read lock. Tests on
8 threads show roughly a 2% performance increase with this.