Switch to using `region` in place of `datacenter`

I think that the owner of the write pipeline should be called a region, not a datacenter. In my opinion, a datacenter is an implementation detail of how hosts are organized and isolated. The term region seems to more generally describe the situation that keeps a number of hosts in a low-latency singe geographic location – and this situation is generally what we want to describe, am I correct?

For reference, see the name of primaryDatacenterKey in the recent commit to master. It could be that what is described here is, in fact, a datacenter, but then maybe we should be thinking about where we want this feature to go in the future?

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