Procedures: Add transaction API #3515

impl ProcedureContext {
    /// Acquire a mutable transaction and execute `body` with read-write access to the database.
    ///
    /// If the `body` returns `Ok`, the transaction will be committed and its mutations persisted.
    /// If the `body` returns `Err`, the transaction will be rolled back and its mutations discarded.
    ///
    /// Regardless of the transaction's success or failure,
    /// the return value of `body` is not persisted to the commitlog or broadcast to subscribed clients.
    /// Clients attribute mutations performed by this transaction to `Event::UnknownTransaction`.
    ///
    /// If the transaction fails to commit after `body` returns (e.g. due to a conflict with a concurrent transaction),
    /// this method will re-invoke `body` with a new transaction in order to retry.
    pub fn with_transaction<Res, Err>(&mut  self, body: impl Fn(&TxContext) -> Result<Res, Err>)) -> Result<Res, Err> {
        todo!()
    }
}