[TINKERPOP-2416] MultiIterator should implement AutoCloseable

[rdtr]

As described in the JIRA, currently MultiIterator does not implement AutoCloseable. This potentially introduces resource leak when we have multiple iterators that implement AutoCloseable and combine them using IteratorUtils#concat. The caller may expect all iterators to be closed automatically by calling CloseableIterator#closeIterator but right now MultiIterator doesn't implement AutoCloseable so even the underlying iterators are auto-closeable, they are never be closed.

This simple PR just adds AutoCloseable implementation to MultiIterator, close all inner iterators if they implement AutoCloseable.

[spmallette]

There are a number of places in TinkerPop code that handle MultiIterator instances. I guess we don't need to explicitly close() those as I don't think they hold expensive resources open (that I am aware of). Any thoughts on that?

[rdtr]

I agree that we don't need to explicitly call close in TinkerPop's code.

The change is mainly for providers. When overriding TinkerPop's API to retrieve elements from their own internal Storage layer, if the returned iterator holds any reference which must be released in the end, it is the provider's responsibility to call close in their own method.

TinkerPop's base class and TinkerGraph doesn't have the resource issue so we don't need to call close there.

[spmallette]

VOTE +1