Raw transactions

[aljazerzen]

Closes #369
Supersedes #370

Adds the following API:

impl Client {
    fn transaction() -> Result<StandaloneTransaction, Error>;

    fn within_transaction(body: FnMut(RetryingTransaction) -> Future<...>) -> Result<...>;
}

impl Drop for StandaloneTransaction {
    fn drop(&mut self) {
        self.rollback();
    }
}

Both RetryingTransaction and StandaloneTransaction implement DerefMut<Target = Transaction>, which gives access to all query_* and execute methods that reside on the Transaction.

[aljazerzen]

Docs screenshots:

[tiberiusferreira]

I'd love to see this merged. Any chance this will be on the next release?

[aljazerzen]

Yeah, possible. It is basically done.

But @tiberiusferreira, can you take a look at this?

#369 (comment)