Adding paging method to query service store iterator

[opudrovs]

Closes #3044

• Added Page methods to iterators.

• Re-ordered iterator methods for consistency.

• Updated iterator fakes.

• Added tests for pagination in iterators.

Notes:

• The order or objects returned by i.result.Hits in indexerIterator, unlike sqliterator is mostly reverse (to the order the objects are seeded) and slightly inconsistent (this part needs to be re-checked, but I saw items for the first page returns in the order of name-7, name-6, name-5 or name-7, name-5, name-6).

• Because of different behavior of sqliterator and indexerIterator, which implement the same Iterator interface, when calling the Page method for sqliterator, you need only to specify the initial offset, the iterator will advance its index automatically by calling Next. But with indexerIterator, you need to specify the offset each time when calling the Page method.

But the above-mentioned issues are of no concern to using Page methods during cleanup. If we need different iterator behavior later, we can research it further and rework it in another issue.

Testing:

• The desired behavior of pagination in sqliterator and indexerIterator is described in tests. For now, just run

go test -v ./pkg/query/store

to make sure pagination works as expected.

[jpellizzari]

Very thorough. Excellent work!

[opudrovs]

@jpellizzari thanks a ton! 🎉