Allow multiple iterators in a single Read / Write transaction

[timshannon]

Currently you can only have one iterator on a read / write transaction:
https://github.com/dgraph-io/badger/blob/master/iterator.go#L396

I have a scenario where I would want to read the consistent state of the current transaction before it's committed. I'm currently getting around this by re-seeking a single iterator, but the ideal situation would allow for using multiple iterators.

According to this comment: #704 (comment) it may be possible.

[dotnwat]

This would be useful to have. For example, to do basic merge-join in a transaction.

[campoy]

Adding this to the list of API changes we're considering for v2.

[campoy]

It seems we could use multiple iterators as long as it's not concurrently, would that be enough or do we need to change anything in our code?