Block-nested loop

[kostja]

Change the built-in graphql execution algorithm (the one which works with "accessors") from simple nested loop to block-nested loop.
https://en.wikipedia.org/wiki/Block_nested_loop

[Totktonada]

After #98 we have breadth-first search executor, which reduces count of network requests (it fetches tuples in blocks), but don't changes execution order, so we need to store the entire data until the end of a request. I filed #236 about that.

[Totktonada]

I'm going to archive the repository. I'll proceed as follows:

• Mark all open pull requests with the archived label and close.
• Mark all open issues with archived and wontfix labels and close.
• Archive the repository.

Consider the following projects:

• https://github.com/tarantool/graphql
• https://github.com/tarantool/graphqlapi
• https://github.com/tarantool/graphqlide