Connection index loss during accessor constructing

[SudoBobo]

Our current formats of connections (both simple and union) have 'index_name' field representing the index that should be used in fetching data from destination collection. In some cases this index is lost during accessor constructing (in accessor_general/build_connection_indexes):

    for collection_name, collection in pairs(collections) do
        for _, c in ipairs(collection.connections) do
            connection_indexes[c.destination_collection][c.name] = set_connection_index(...)

Here is the case: consider A → C and B → C connections with the same names. Only last one will be saved in the connection_indexes structure and A → C connection can fail during execution.
Another case is when different variants (in union connection) points to the same destination collection.

    for collection_name, collection in pairs(collections) do
        for _, c in ipairs(collection.connections) do
            for _, v in ipairs(c.variants) do
                connection_indexes[v.destination_collection][c.name] = set_connection_index(...)

[Totktonada]

Dupe of #70?

[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