Add support for Standalone transactions and explicit commits and rollbacks

[tiberiusferreira]

Adds support for explicit user commits and rollbacks, while still allowing retryable transactions. Also allows users to rollback in retryable transactions, albeit the user now has to explicitly commit them too.

async fn retryable_transaction() -> Result<(), edgedb_tokio::Error> {
    let conn = edgedb_tokio::create_client().await?;
    let val = conn
        .retryable_transaction(|mut tx| async move {
            let new_value = tx
                .query_required_single::<i64, _>(
                    "
                    WITH C := UPDATE Counter SET { value := .value + 1}
                    SELECT C.value LIMIT 1
                    ",
                    &(),
                )
                .await?;
            tx.commit().await?;
            Ok(new_value)
        })
        .await?;
    Ok(())
}

Standalone transactions:

async fn transaction() -> Result<(), edgedb_tokio::Error> {
    let conn = edgedb_tokio::create_client().await?;
    let mut tx = conn.transaction().await?;
    let new_value = tx
        .query_required_single::<i64, _>(
            "
        WITH C := UPDATE Counter SET { value := .value + 1}
        SELECT C.value LIMIT 1
    ",
            &(),
        )
        .await?;
    tx.commit().await?;
    Ok(())
}

Closes: #369

[edgedb-cla]

All commit authors signed the Contributor License Agreement.