geo: Add support for finer-grain levels of locality

[es1024]

Jira Link: DB-922

Description

Context: #9980

We currently only support global and region-local transactions. This means that if we have a cluster spanning much of the globe (e.g. nodes in the US, EU, and Asia Pacific), with some data replicating across only multiple regions within the US, distributed transactions to data within the US will incur global latencies at commit time due to the use of the global transaction table.

This can be handled by extending the notion of locality to be more fine-grained than region-locality. In the example above, we could have a multi-region transaction table over the multiple regions of the US to be used for transactions operating on data in more than one US region, but no data outside of the US.