cdc-sink perf subcommand

[bobvawter]

It should be straightforward to have a standard performance benchmark command that will characterize the performance of a user's staging and target databases. This command should exercise the stage and apply packages, as well as drive the cdc.Handler type. By testing these three (at least) parts of the system, we can understand where major bottlenecks are likely to occur. Even if it's not possible to generate data to simulate the user's exact schema, we can still get a sense of best-case QPS.