Before, if we had parent transaction A, and two nested sibling transactions
inside it: B1 & B2, knex would hang when you asked it to execute any kind of
a database operation in the second child transaction.

Also, it had a bad Promise.settle() call that would break in case the
previous child transaction promise not resolve to an array, which typically
(e.g. when calling trx.commit() on the passed transaction object or
resolving the transaction promise with no value) it would not.

- removed an unnecessary `||` with an always `undefined` value
- avoided waiting for an always resolved promise

The only thing each transaction needs to track is its last direct child
transaction. That is then used to prevent each sibling transaction from
starting (i.e. returning its connection) before its predecessor
transaction completed.

- improved related comments
- renamed `trx._queue` to `trx._previousSibling`
- made code waiting for `trx._previousSibling`, instead of code initializing
  that promise, more explicit about waiting for the promise to be either
  resolved or rejected, i.e. `settled`/`completed`
- made the code a bit more compact