[keyserver] move async iife to standalone function

Summary:
keyserver.js is already massive, and the async iife in the middle makes it harder to follow

Depends on D13667

Test Plan:
cleared the access token from the metadata table and ran keyserver as a secondary node. confirmed that node polled every 5 seconds for identity info.

killed and ran keyserver as primary. access token was added to metadata table.

killed and ran keyserver as secondary again. node was able to fetch access token and did not get stuck in loop

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13694