Add a `TransactionResult` type.

[jmalloc]

The transaction result would be returned from Transaction.Commit() on success, and include the offset of the first event saved within this transaction, if any.

[jmalloc]

Rather than putting the offset in this new result type, we should place the same "item" slices (not the parcel slices) that appear in pipeline.Response, that will help with #210 and eventually those slices can be removed from pipeline.Response.

[danilvpetrov]

So generally the Transaction interface inside persistence pkg would look like the following (including the new TransactionResult type):

// Transaction exposes persistence operations that can be performed atomically.
// Transactions are not safe for concurrent use.
type Transaction interface {
    aggregatestore.Transaction
    eventstore.Transaction
    queuestore.Transaction
    offsetstore.Transaction

    // Commit applies the changes from the transaction.
    Commit(ctx context.Context) (TransactionResult, error)

    // Rollback aborts the transaction.
    Rollback() error
}

// TransactionResult is the result returned by Transaction.Commit() method.
type TransactionResult struct {
    EventItems []*eventstore.Item
    QueueItems []*queuestore.Item
}

Is this what you're thinking?

[jmalloc]

Yep, exactly!

[danilvpetrov]

Fixed by #235.