Improve processing of broker data store responses

Now retrieves and processes all N available responses at once instead
of one-by-one-until-empty.  The later may be problematic from two
points: (1) hitting the shared queue/mailbox matching logic once per
response instead of once per Process() and (2) looping until empty is
not clearly bounded -- imagining a condition where there's a thread
trying to push a large influx of responses into the mailbox while at
the same time we're trying to take from it until it's empty.

src/broker/Manager.cc

@@ -936,11 +936,15 @@ void Manager::Process()
 
 	for ( auto& s : data_stores )
 		{
-		while ( ! s.second->proxy.mailbox().empty() )
+		auto num_available = s.second->proxy.mailbox().size();
+
+		if ( num_available > 0 )
 			{
 			had_input = true;
-			auto response = s.second->proxy.receive();
-			ProcessStoreResponse(s.second, move(response));
+			auto responses = s.second->proxy.receive(num_available);
+
+			for ( auto& r : responses )
+				ProcessStoreResponse(s.second, move(r));
 			}
 		}