Improve refreshMetadataIfNecessary in BrokerPool

[mkocsar]

The performance problem which this PR addresses is described in Performance degradation when sending messages to thousands of topics.

Functionality-wise the updated code is identical to the original, but it has O(n log n) runtime complexity instead of O(n^2) which becomes relevant if a client publishes to multiple thousands of topics.

Measured improvement (using mkocsar/test-send-duration.js):

Topics	'send' duration (original)	'send' duration (updated)
1	2.080 ms	1.835 ms
10	2.161 ms	1.999 ms
100	2.760 ms	2.056 ms
250	3.947 ms	2.212 ms
500	6.145 ms	2.238 ms
1000	15.741 ms	2.658 ms
1500	18.794 ms	2.981 ms
2000	19.272 ms	3.389 ms
2500	19.718 ms	3.853 ms
3000	23.959 ms	4.299 ms
3500	31.592 ms	4.286 ms
4000	41.728 ms	4.692 ms
4500	53.501 ms	4.864 ms
5000	65.370 ms	5.047 ms