repo-updater: Fix error log spamming
Created by: tsenart
Some of the external service connections syncing threads were being
started through init
functions and, hence, weren't waiting for the
frontend API to be available before sending requests its way.
This led to confusing log spam which could have given new users a bad first impression.
This change-set addresses this problem by explicitly starting external service connection syncing "threads" for all external service kinds.
Additionally, we also wait for gitserver to be available, which was also causing some logged errors.
Partially addresses #1910