* Fix data race for max connection limiting in proxy directive.

The Conns and Unhealthy fields are updated concurrently across all active
requests.  Because of this, they must use atomic operations for reads and
writes.

Prior to this change, Conns was incremented atomically, but read unsafely.
Unhealthly was updated & read unsafely.  The new test
TestReverseProxyMaxConnLimit exposes this race when run with -race.

Switching to atomic operations makes the race detector happy.

* oops, remove leftover dead code.