proxy_cluster_post_request lock delay under load

Under load, proxy_cluster_post_request has shown large delays on a lock. That seems from waiting for the lock to be released by the longer internal_find_best_byrequests calls in MODCLUSTER-757. We may be too eager in requesting the lock in proxy_cluster_post_request and so could potentially avoid this delay and contention with internal_find_best_byrequests in typical configurations (no error status).