Expose DB transaction mechanism

[mempirate]

Various parts in the codebase require atomicity:

bolt-registry/src/sync/mod.rs

Lines 282 to 293 in c901226

	// TODO: retries on transient faults (e.g. network errors)
	// TODO: use a DB transaction here
	if let Err(e) = self.db.register_validators(&registrations).await {
	error!(error = ?e, "Failed to register validators in the database");
	}
	for operator in operators.into_values() {
	if let Err(e) = self.db.register_operator(operator).await {
	error!(error = ?e, "Failed to register operator in the database");
	}
	}
	}

bolt-registry/src/sync/mod.rs

Lines 182 to 185 in c901226

	// TODO: retries on transient faults (e.g. network errors)
	// TODO: ideally all DB operation in an epoch transition need to be made
	// in a single transaction to avoid partial updates in case of failure
	self.db.update_sync_state(state).await?;

We should expose a transaction mechanism at the correct level of abstraction.