support for DeleteRange

[ekovacs]

currently if i know that all the items in a given range sould be removed, i could only iterate through the Results and remove them one by one.

It would be great if there was support for a range delete, that could get a FilterIterator (which wraps a memdb.ResultIterator that is returned by either a txn.LowerBound(...) or a txn.ReverseLowerBound(...) )

the results which the filterIterator would yield could be removed in a batched /efficient manner.