Use associative arrays by default for MySQLi results

Summary:
The MySQLi extension supports three different fetch modes
to represent a row as an array:

• associative keys
• numeric keys
• both of them

This change switches from the default "both of them"
to associative keys to encourage more expressive code,
and ease code maintenance.

The queryScalar method will seek the first key of the array accordingly.

Test Plan: Unit tests added and updated accordingly for MySQLi

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3207