MEDIUM: threads/lb: Make LB algorithms (lb_*.c) thread-safe

MEDIUM: threads/lb: Make LB algorithms (lb_*.c) thread-safe

A lock for LB parameters has been added inside the proxy structure and atomic
operations have been used to update server variables releated to lb.

The only significant change is about lb_map. Because the servers status are
updated in the sync-point, we can call recalc_server_map function synchronously
in map_set_server_status_up/down function.