Use ResultClass::HashRefInflator instead of building array on iteration

This retrieves a projection of the required columns into the instant
answer list array, minus the topics currently.

On my test environment, run time for this query goes from ~700ms to ~70ms.
Remove unused columns from IA index JSON, prefetch topics.
We no longer populate the topic array, get topic names from instant_answer_topics
Merge pull request #359 from duckduckgo/ia-index-performance

Use ResultClass::HashRefInflator instead of building array with iteration