Move timeseries schema to CockroachDB

[bnaecker]

The timeseries schema are currently stored in ClickHouse, alongside the timeseries data itself. That's less than ideal, given ClickHouse's lack of consistency guarantees. We should move that to CockroachDB to have consistent operations around creating timeseries schema and updating their versions (see #1293). This would support operations such as registering not just as a metric producer, but as a producer of a particular timeseries version.