optimize resolve routine

[oliver-sanders]

• The resolve routine was an async recursive function.
• Because this function gets called for every returned value.
• As a result, the number of async tasks can become extremely large for large or deeply nested GraphQL responses.
• Python functions have overheads, async tasks have additional overheads on top, this created a severe performance bottleneck.
• Simple solution, make the function iterative rather than recursive.

For an example usage in my project, this brought runtime down by ~10 seconds.