Only add handlers for pausing/resuming metric receipt when USE_FLOW_CONTROL is enabled

MetricReceiver calls `addHandler` twice per incoming metric connection; `addHandler` is a fairly expensive method which is O(n) in the number of registered handlers (since it walks through an array of **every** registered handler). We profiled our very busy carbon-caches with cProfile and found that the addHandler calls were something like 4% of our total CPU time, and were sort of pointless because we have flow control disabled.

This patch gets rid of the pauseReceivingMetrics handlers when USE_FLOW_CONTROL is disabled and gets addHandler out of our cProfiles. Obviously, 4% is a pretty negligable perf improvement, but it seemed worth upstreaming anyway.