[keyserver] Stop creating GENESIS_PRIVATE thread on account creation

Summary:
This addresses ENG-9411, although the following diffs are necessary for a related follow-up: making it possible to create a PRIVATE thread.

Depends on D13534

Test Plan: Confirm that when a new account is registered, a GENESIS_PRIVATE thread is not created for them.

Reviewers: tomek, kamil, inka

Reviewed By: inka

Differential Revision: https://phab.comm.dev/D13535