Add debug transactions to the C API and fdbcli

From within FDB itself, one can call `NativeAPI::debugTransaction` to set a debug identifier for a transaction. When committed, this identifier triggers logging when each part of the transaction authority encounters it, which is helpful in diagnosing slowness in misbehaving clusters.

Currently, there's a background transaction that runs every 10s that enables this logging. If a user wishes to tag specific transactions of their own, there's no way to do so.

There's not really a great reason that I'm aware of to hide this from clients, so we should probably add it to the C API, so that it can be set via bindings, and to fdbcli.