DbKVS Cannot Just Upgrade To _Transactions2 Without Further Checks

[jeremyk-91]

There was an irregularity in how transactions and the coordination service work for users of these key-value services, which was exposed in #4070. Specifically:

(having transaction versions and the coordination service disagree) can lead to SEVERE DATA CORRUPTION™️ if we end up subsequently releasing a version of DbKVS that does support transactions2, as in #3869, since everything has actually been written at version 1 despite whatever the coordination service says is correct.