Introduce a Transaction object, now with a TransactionDefinition

To move forward with the Transaction Object, here is an alternative proposal to #571.

This keeps the same basic idea, but now `createTransaction()` returns a `TransactionDefinition` object, which is configurable, and has a `begin()` method that starts the underlying transaction and returns the `Transaction` object:

    $tx = $em->createTransaction()                                 // TransactionDefinition
        ->withIsolationLevel(Connection::TRANSACTION_SERIALIZABLE) // TransactionDefinition
        ->begin();                                                 // Transaction

I think that this implementation checks all the boxes:
- Clear separation between the Transaction and its Definition
- Once the Transaction is created, its Definition is set and cannot be changed
- No risk to forget to call `begin()`: if you do, you'll deal with a TransactionDefinition and just get a *call to undefined method* if you try to `commit()` it. Plus, your IDE will be able to warn you while coding.

And needless to say, we're still keeping 100% BC compatibility.

What do you think?
So far, I've considered the `TransactionManager` to be a lower-level class, that we should not have to access directly, as I thought it might be cumbersome to have work with a reference to the `TransactionManager` in addition to the `Connection` in our apps. But I theoretically agree with you. What do others think?