MINOR: lb/map: use seek lock and read locks where appropriate

MINOR: lb/map: use seek lock and read locks where appropriate

- map_get_server_hash() doesn't need a write lock since it only
  reads the array, let's only use a read lock here.

- map_get_server_rr() only needs exclusivity to adjust the rr_idx
  while looking for its entry. Since this one is not used by
  map_get_server_hash(), let's turn this lock to a seek lock that
  doesn't block reads.

With 8 threads, no significant performance difference was noticed
given that lookups are usually instant with this LB algo so the
lock contention is rare.