Make centroidSet safe for concurrent access

[spenczar]

It's easy to imagine the use cases - you might have many goroutine workers processing data and want them to centralize their measurements.

One solution to this is to give each goroutine its own TDigest and add the ability to merge them. This has the advantage that it requires minimal locking.

A direct, blunt-force approach would be to lock access to the slice of centroids. This seems much less good.