MINOR: lb/first: use a read lock in fas_get_next_server()

MINOR: lb/first: use a read lock in fas_get_next_server()

The "first" algorithm creates a lot of contention because all threads
focus on the same server by definition (the first available one). By
turning the exclusive lock to a read lock in fas_get_next_server(),
the request rate increases by 16% for 8 threads when many servers are
getting close to their maxconn.