Introduce the ability of awaiting for eager beginning a Transaction

[bigmontz]

The current implementation of session.beginTransaction does not wait the success message. This kind of behaviour can cause future calls to run and commit because of non-reported errors during the begin.

For solving this issue, the transaction object return by the session.beginTransaction was changed to TransactionPromise. This new type holds all the behaviours from a Transaction and also holds the promise for the beginned transaction. This way, the caller can eagerly eager check the transaction begin and at the same type keep the API existing in the current codebase.

Eager Mode:

const tx = await session.beginTransaction() // waits for begin result
const result = await tx.run('RETURN 1') // only throws RUN related errors, not reachable when begin fails

Lazy Mode:

const tx = session.beginTransaction() // no wait, any begin errors will be thrown
const result = await tx.run('RETURN 1') // throws BEGIN and RUN related errors

[bigmontz]

Depends on: neo4j-drivers/testkit#397