[keyserver][lib] Factor out joinThreadSubscription

Summary:
This diff does two minor things:

1. Factors out joinThreadSubscription so it can be reused in the following diff
2. Redefines defaultSubscriptionString so that it uses defaultThreadSubscription instead of redefining it

Depends on D12781

Test Plan: Flow

Reviewers: tomek, kamil, marcin

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12824