Add functionality to Lisk to only get some columns from the database

Summary:
Added loadColumns, loadColumnsWhere instance methods to Lisk, so when you only
need some fields of your object loaded, you can do so. This will be useful for
places where we fetch a large number of rows, but only care about a few columns.
In that situation, these functions can be used so the db doesn't have to return
as much data.

Test Plan:
Loaded a typeahead to check that the existing lisk functions still work.
Modified typeahead to fetch data using loadColumns instead of loadAll and
checked that it still works.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley, nh, jungejason

Differential Revision: 947