Fix pagination + multipart indexes

[mRrvz]

Fixed invalid results for pagination queries when filtering on
a parts of a composite index. Now, instead of "scrolling"
to the desired tuple (during pagination) using the
scroll_to_after_tuple function, we change the iterators
(EQ -> GE, REQ -> LT) to use them in
index:pairs function, together with a combination with
additional filtration. Closes #170

[ligurio]

@mRrvz Alexey, thanks for the patch!

Few comments:

• could you add explanations to commit messages "Fix unnecessary sharding key", "Remove unnecessary test". For me it is not clear what was fixed regarding to sharding key and why we need to remove test.
• I suppose the item to changelog should be added in the same commit where fix was added.

[olegrok]

Thanks for your patch. Please resolve the rest of not my comments.

[dokshina]

Please make Changelog more informative.